I'm not talking about Core Rules here. I just want to point out that system development is a reaction to obstacles encountered while running the system. And while a DM ruling that anyone can try to climb a tree is perfectly fine with me, the special ability of thieves who can climb sheer surfaces is something different, and requires a modification to the existing system, which in turn forces a change to the game world.
I guess, the thief class was invented solely for the purpose of working around an obstacle, or encounter that otherwise would prove very difficult. Hence, the added thief class brought balance to the system, didn't it?
Actually, I don't know. I just came up with the cartoon, and wrote down some thoughts it triggered.
*On a side note: I think the caption: "If you can't climb, you've got to come around the long way, slay the dragon and get past my parents ..." is more subtle, and might work better. But then, I've never experienced my gamers to get this kind of subtlety. I was even thinking about "[...] and sneak past my parents ..." Anyway. Move along, play a game.