I'm pretty sure FLAC and maybe all other popular compression formats use M/S by default.

• Biased observation: people read "joint stereo" and believe that it will join the channels more closely, that dual mono is the right thing, blah blah blah. Will not hold up to a blind test.
• Compressing two channels in a lossy way means different losses per channel. Channels might sound more distinct because the lossiness introduces artefacts that make it sound that way.
• Joint stereo means the encoder can save some bits, and then it will re-evaluate the "how to get best sound for cheap". If it can get overall better sound by ever so slightly narrowing down the stereo image, then that is the right thing to do, and if you don't like it, increase bit rate. Even if it saves bits that can be used to improve, it isn't given that absolutely every aspect is getting better (or at least not worse).
  
• Even if the previous point gives more room for improvement, then it is based on a model of reality - and sometimes that fails.

IMO the export panel of an audio editor shouldn't display all the options for something like LAME. That would look messy,

confuse most users and generate support tickets.

Power users who want to tinker with weird settings have other ways to do so.

But I admit that I didn't look too much into the difference between "forced" and "joint" stereo. If there is a realistic scenario that we need to worry about (audible bad consequences), your point is valid. Otherwise, not really.

you've proved my point.

Surely anybody who uses an audio editor will have at least a basic knowledge of lossy and lossless codecs, otherwise they'd be wasting their time. The idea of over simplifying encoder interfaces to cater for those who don't know what they're doing is counter productive. Most users of audio editors know precisely why they're using them and want all of the options offered by the various codecs to be available. Or, am I missing something?

I'm not sure about that, because the more I learn about "forced stereo", the less I want it to come back to Audacity.

Joint Stereo attempts to reduce the data by exploiting the similarities between the channels, but if the channels are totally dissimilar there is no reduction, and decoding may introduce crosstalk.  Therefore, if either is to be despised, surely that should be Joint Stereo?

This exact misconception is the reason why the option should be removed. Selecting joint stereo does not force mid/side coding! If the two channels are dissimilar, they will be coded separately, exactly as they would when choosing forced stereo.

In normal “Stereo” mode, MP3 stores a separate Left and Right channel, though bitrate can be distributed between Left/Right as needed. In “Joint Stereo” mode, there are still two channels, but they are called Mid and Side. Before the compression, Mid/Side are computed from Left/Right. While Mid stores the common portion of the Left/Right channels, Side stores the difference. After decompression, Left/Right will be reconstructed from Mid/Side.

The advantage of “Joint Stereo” is that, usually, the Left/Right channels are pretty similar, so most information will be in the Mid channel and only very few information in the Side channel. This means that, with “Joint Stereo” mode, the redundant part doesn’t have to be stored twice! This means that “Joint Stereo” can use the available bits more efficiently than normal “Stereo” mode does - provided that the Left/Right channels are somewhat similar.

The rule of thumb is that “Joint Stereo” is advantageous at lower MP3 bitrate, while it’s less helpful on higher bitrates. Also the advantage depends a lot on the content. Anyway, with the LAME MP3 encoder you don’t need to worry. It uses “Joint Stereo” by default (-m j 2), but still switches between Left/Ride and Mid/Side mode dynamically, i.e. it always picks the “best” mode for every frame. You can force Mid/Side mode (-m f 2), but that’s not recommended…

LAME - Mid/Side Stereo

During years, what is called Joint-stereo has been misunderstood.
Joint stereo in MP3 is a mechanism to selectively choose between three modes of storing stereo information. These three modes are Simple Stereo , Mid-Side Stereo, and Intensity-Stereo.

In Simple Stereo, the encoder analyzes the left and the right channels independently and stores the information as-is, without further checking the similarities in the signal¹

In Mid-Side Stereo, the encoder analyzes the left, right² , mid (l+r) and side (l-r) channels. It then gives more bits to the mid than the side channel (as usually the side channel is less complex) and then stores just the mid and side channels into the resulting MP3.
This way, the mid channel can be encoded as if the frame was bigger, and as such have more quality with the same bitrate.
Note: Mid/side in MP3 is switched frame-by-frame. In AAC, it can be switched band by band.

Intensity-Stereo (not supported in LAME) uses a technique known as joint frequency encoding, which is based on the principle of sound localization.
Human hearing is predominantly less acute at perceiving the direction of certain audio frequencies. By exploiting this 'limitation', intensity stereo coding can reduce the data rate of an audio stream with little or no perceived change in apparent quality.
It works by merging the upper spectrum into just one channel (thus reducing overall differences between channels) and transmiting a little side information about how to pan certain frequency regions.
This type of coding does not perfectly reconstruct the original audio because of the loss of information and can cause unwanted artifacts. However, for very low bitrates this tool usually provides a gain of perceived quality. ³

The LAME mid/side switching criterion, and mid/side masking thresholds are taken from Johnston and Ferreira, Sum-Difference Stereo Transform Coding, Proc. IEEE ICASSP (1992) p 569-571.

The MPEG AAC standard claims to use mid/side encoding based on this paper.

1.  This is not the same than dual-mono. Dual-mono should be used where the left and right channels of the input file contain two different streams, where you should choose one (as in two different languages)
2.  If one channel has much less noise masking in a certain band than the other, it could happen than the noise spread (by mid/side stereo) may no longer be masked for that channel. If both channels have the same masking, then the noise spread between both channels will be equally well masked.
    To prevent this from happening, there is an analysis done on the left and right channel to determine the noise masking thresholds and properly mask the noise.
3.  Quote from wikipedia Joint_stereo.

Then why not offer a similar scenario to foobar? Simplified VBR and CBR interfaces for the uninterested/uneducated (I'm not trying to be patronising!), but with the option to expose the command line for those who know what they're doing and have specific requirements?

I'm not sure about calling it Forced Stereo, Full Stereo maybe.

the OP wants suggestions for persuading enthusiasts that omitting the Full Stereo export option is a good idea.  I say that's barking up the wrong tree.  Some people might never use it, but enthusiasts are never going to be happy with a reduction of functionality (and an unnecessary reduction so far as I can see).  Whose bright idea was that???

Are you saying there is no benefit to full stereo, or almost no benefit?

Interesting, but does that matter if that's what the user wants to do?  This sounds like the argument of why bother with this codec or that codec when xxx codec is "clearly" superior.

I see the export pane in the "what's new" information, and it casually labels (what you say is) Joint Stereo as "Stereo".  What's more, I can't see a button leading to advanced options where the option to select separate stereo is available, so how would I do that if I wanted it?

I suggest that "getting some users angry" is not a good path to go down just for the sake of making things simpler for the inexperienced – educate the inexperienced rather than try to defend what you've done by flowery arguments which will not persuade power users.

Surely anybody who uses an audio editor will have at least a basic knowledge of lossy and lossless codecs, otherwise they'd be wasting their time. The idea of over simplifying encoder interfaces to cater for those who don't know what they're doing is counter productive. Most users of audio editors know precisely why they're using them and want all of the options offered by the various codecs to be available. Or, am I missing something?