I believe you're forgetting something. Not only there are several mp3s that ARE bad equalized and not normalized to have same "volume" as the others you have, but also you may wanto to alter the way an mp3 sounds for personal reasons that are totally independent from "quality" of reproduction.
As to the first aspect, consider that, may one consider that right or wrong, many, including me, have MP3s without having the original recording, and that is essentially because as others pointed out it is not always easy to find original of old and rare songs, or simply because a friend gave it to you and you never bothered to buy the cd. Also, sometimes old original were badly recorded and you many need adjustements that are not needed on most of the other mp3s you may have.
As to the second, consider that you may want to listen to audio books, or othrer source, that, for one reason or another, may require, or just make you appreciate, some sort of adjustment. Not to mention that you may want to alter the way an mp3 sounds for "creative" or "personal" reasons (once I used my empeg as a source for "special effect" while playing an RPG with friends, just to create a nice atmosphere. In that case, I would have even liked some effect such as evironmental dsp.) As you can see, this has nothing to do with the intent of reproducing music at high quality strandards (at which regard you are totally right), but with simply the personal listening freedom and wide range of usage of each single user. Thinking of an EQ or an evironmental DSP only in terms of reproduction quality is quite restrictive, IMNSHO, and obviously with such an incredible tool as the empeg one is naturally driven to think of all the possible applications.

Finally, having per-song eq settings does not mean you have to personalize ALL your mp3s. You would use that just for those you think need that.