And what about the people who listen to tunes while jogging or biking? How am I gonna fit a pair of Koss earcup headphones under my bike helmet? Or what about the people who listen to tunes at work? How are they gonna hear their phone ringing if they're wearing those fancy over-the-ear cups? And what about the people who suffer from any particular degree of hearing loss? How are they gonna adjust the frequency response to a profile that's acceptable to them without an equalizer? The finest headphones in the world won't give them what they need...an equalizer is exactly what they need.
Sitting at home in the BarcaLounger with a high-dollar set of audiophile headphones is not how you're gonna find most people listening to tunes on their Hero...they're most probably gonna be on-the-go, just trying to get in a few tunes any way they can.
If you do own a high-dollar set of audiophile headphones, and you do sit at home in the BarcaLounger listening to music on your cellphone, then IMO your priorities are all wrong, dude. Sorry, but your (paraphrased) statement "if you have a perfect pair of headphones with a perfectly flat frequency response, and if you have perfect hearing, then you wouldn't need an equalizer" is rather lame, because the perfect pair of headphones doesn't exist, and no one has perfect hearing, and few people are sitting perfectly still and solely focused on the music while they're listening to any portable music source.