You can have "uniform mechanics" (without going to the extreme of 4e) and still support a proper "class" based system.
Anybody can stand behind a column IN SHADOWS. The thief picks the darkest, least suspicious shadows quickly and instinctively. That's why he's better at it.
I think its a matter of degree. The further you get away from uniquely determined class-linked abilties by appealing to a single mechanic the closer you are to something like GURPS. Which in itself is not bad if that's what you are looking for. But we play a class based game for a reason.
Single mechanic games have to make an effort to distinguish classes and keep them separate. I believe unique XP tables are one of the better ways to enforce this. It really doesn't affect in-game play after all, just after-game levelling up. And to complain about looking something up kind of defies reason in light of the number of class based tables DCC has built into it's system.
In terms of a cost benefits analysis a universal xp table loses alot more than it gains.
And the first quote about "anyone hiding in shadows" is exactly what a single mechanic game encourages. The roll is the same it's just that the thief has a better chance at it. It blurs the distinction. As opposed to the thief having a separate roll and or table to refer to what is obviously distinct from what the fighter does. Sure the fighter can make a wisdom or dex check to try something, but he doesn't have access to the same kinds of rolls/tables as the thief.
Keeping things distinct reinforces a class based system. Universalizing it blurs those distinctions. There's a reason GURPS was called "universal".
Looking at it in this way helps you see what Goodman may have been tryin to do. Joseph explained in the designer's notes that he was trying to make a game that "might have been" if modern game design principles had been around back in the day. So what he does is use a single mechanic (which is built upon d20, not a simple d20 system--its' truly revolutionary) and class based tables that set up and define uniqueness for each class. The mechanic is the same for actual play, but each class is clearly separated the way the game is written. Unique XP tables I see as a part of this.