Thieves...

Ya see, you're all going about this all wrong.

Thieves have blue hair. Not only that, but, if you try to envision it in a higher graphics setting, it's a pretty badass style.
Thieves look totally badass. And so what if they'd rather run than let their badass hair get messed up.
Black Belts are plain ugly. So they don't compare.
Ninjas are ugly. And they don't seem to run as well. (Or maybe enemies in the later game are harder to run from?)

But thieves, they're like the Elvis of FF1. I wouldn't sacrifice one for anything.