"Don't confuse Joint Stereo (Stereo mode 1) with the Joint Stereo coding used for MPEG layer 2 encoding - it is not the same. Joint Stereo (Stereo mode 1) encoding for MPEG layer3 allows the XingMP3 Encoder to use additional methods of encoding, specifically - MS Stereo (Middle/Side Stereo), and for lower bitrates only, Intensity Stereo, in addition to the Independent Channel coding used for Stereo mode 0. MS Stereo uses one channel to encode information that is identical on the left and right channels and the other channel to encode the differences between the two channels. Intensity Stereo encodes only bits that are perceived to be important to the stereophonic image. The XingMP3 Encoder uses Intensity Stereo only in low bitrate files, (96kbps or less) where file size is critical to the user. In Joint Stereo (Stereo mode 1), the encoder dynamically (frame by frame) chooses the method of encoding that produces the best quality for each individual frame. Dynamic encoding improves compression efficiency which results in a higher quality file using less bits. Stereo mode 0 encodes the left and right channels independently. The total bitrate remains constant, but the split between the channels can vary. The XingMP3 Encoder uses this flexibility to improve quality by allocating more bits to the channel with the more dynamic signal. For MPEG layer 3 encoding, Stereo mode 0 limits the encoder to only one method of encoding - Independent Channels. Because Stereo mode 0 is limited to one method of encoding, Joint Stereo (Stereo mode 1) in most cases produces higher quality. In the exceptions, the Stereo mode 0 quality will be essentially equivalent to Joint Stereo (Stereo mode 1)."
The technique of manipulating stereo audio signals in Mid/Side format was devised solely for the purpose of enhancing audio compression. As far as uncompressed audio is concerned, it offers no benefits whatsoever. On the other hand, neither does it have any drawbacks - other than the fact that it is slightly more complex to handle than simple stereo.