What fun is it to have everything unlocked at the beginning? Unlocking stuff is fun and gives you incentive to play a lot more.
Playing fighting games against the computer is like playing tennis against a wall. Maybe its fun when you start out, trying to learn the moves, but pretty soon you want to play against humans, and once you do the wall is insanely boring, and not even helpful from a learning standpoint.
Ive logged I think 27 hours on SF4 so far, the first two were dedicated to unlocking characters, I have no desire to ever play single player again when playing with people on XBL is so much more fun and rewarding. Seth will only get unlocked if someone else does it on my Xbox, or a fund gets started to coerce me into doing it.
The incentive to play, should be a fun and balanced game that makes you feel like you can and will get better with more practice. Locking characters does the opposite, by forcing the user to do something they may not want to do in order to access all the content. I cant tell you how many times Ive had to unlock all the toons in Soul Calibur on Dreamcast, and each time its as terrible as the first.
Beat em ups are for playing against the computer, fighting games are for the rock paper scissor game against a live breathing opponent.