The incredibly stupid volume cap mandated by Law in the EU makes all your mp3 files replaygained to 89 dB absolutely inaudible in noisy areas (busy streets with a lot of traffic).
The default volume limitation on European iPods is limiting the output to 100dB with stock earplugs.
In both cases - or more precisely any replaygain implementation which respects the 89dB reference - you need some kind of amp-function in the player..... optimally, a hardware-preamp for the headphones, since that would solve the root issue: weak amping of headphones/speakers. Humans have a tendency to just want an "implementation which works" without caring about how efficient that implementation is. If the weak hardware preamping is "fixed" on the software side, then the pressure to fix the actual hardware-problem vanishes. This is the case for both, changing references or on-the-fly amping.... its just that on-the-fly amping is temporary and therefore less destructive.
Yeah, I haven't really done anything with MP3Gain for years now. To be honest, I don't use it myself any more. I joined the iPod crowd, and now I just do a ReplayGain scan with foobar2k, and dump the Track Gain results into Apple's "iTunNORM" tag. Then with Sound Check turned on, the volume on all my stuff is correctly adjusted without having to modify the data.