But there's absolutely nothing in the earliest write-ups of the thief class that suggests that. The thief abilities seem to be selected with the idea of avoiding direct confrontation: pocket an item without anyone knowing, hide in shadows, move without making a sound, attack unawares from behind. They steal when no one is looking, not by outright thuggery.
I think the idea that thieves ought to be better at dodging comes from one or two places:
- Thieves are limited to lighter armor, so maybe it's because they prefer dodging?
- Thieves use Dexterity as their primary ability, and that has something to do with dodging?
But, of course, although many people (including me, from time to time) have used Dex as a stand-in for speed or agility, it's not really either; it's nimbleness of finger and hand-eye coordination. Originally, all Dex modified was accuracy with missile weapons. Later, Greyhawk added the AC modifier, which may have cemented the idea that Dex = dodge skill in people's minds. However, if you are using the AC bonus, then that's your dodge mechanic; thieves aren't naturally better dodgers because they are thieves, but because they may happen to have a high Dex. There's no reason to let thieves double-dip for an ability that isn't really part of the class conception.
Now, if you want to do a class that is all about speed, agility, and unusual movement abilities, you could take the Thief, drop open locks, pick pockets, remove traps, hear noise, and language abilities, and replace those with a move bonus. Hey, that gives me an idea...