Yea, for the casual market it probably would be a big selling point... I won't deny that.
But when you've played like 20,000 matches online and you keep seeing people make the same absolutely horrible characters with little to no creativity, it gets annoying... I only did a bit more then 2,000 matches on soul calibur 4 before i went insane from it.
To be fair though, soul calibur did some incredibly bad things with it. Tons of the costume pieces clipped through each other in a glitchy manner, the clothing destruction made certain choices absolutely ridiculous looking (you could go from full plate armor to underpants in 3 hits), and hitbox detection was a nightmare as every single custom character was different. Certain animations were also completely off because characters did not scale correctly to their original (astaroth/rock custom chars were a good example of this).
It created a lot of problems that made the game look bad from both a graphical point of view, and a competitive point of view.
Until someone really spends the time to fix all of these problems and hammer down a quality create-a-character-mode, I'd rather they just left it out completely. It's too much a part of soul calibur now to just get rid of it, but it is also painfully amateur.
If Team Ninja can up the production value of such a feature, then sure. I don't believe they have the time to do that in the less-then-a-year of development time they probably have on the game though.