While I think it would be nice to have an equalizer for windows phone just to appease the masses I have never honestly seen a need for an equalizer on what is basically a Zune/Ipod/Mp3 player. When you are jamming out to music are you really using your phone/portable whatever? Most people I know have dedicated sound setups, car setups, pc setups for rocking out. They use their phone's player for things like audiobooks, the occasional "mix cd" while working out, and stuff like that.
I personally don't really see a need for an equalizer for MOST digital music anyway but I know other people's mileage varies. I think it really boils down to can this even be done with the way the software is currently setup? I think WP just needs time. iOS was terrible in its infancy (and could be argued still terrible but that's a whole other topic.) Android likewise was just plain bad up until about a year(ish) ago. For only being on their second update to date I would say WP is on par with its older siblings.