Gain isn't the same as normalization, generally it's considered superior. It doesn't require reencoding the files and makes the songs more consistent than normalizing them.
On the other hand, I've applied mp3gain to all my mp3s, but I've yet to come across a decoder that takes advantage of it...
libvorbis on the other hand _does_ use ReplayGain settings. This means that mp3s generally sound louder than oggs.
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