Why let a character gain a power that I don't let him/her exploit fully at all times?
Because I don't know every rule in the game and because I don't have a handle on how every single power can/will/might change my plan for the evening.
That's why. And I can't be the only DM on these boards which hasn't got a perfect grasp of the rules, and who is actually surprised at the effects of some powers, once in a while.
Sometimes, for example when I've done some planning in short order to get a game together because the other DM called in sick or something, I get in situations where I'm non-plussed by what the players do, because of how they use the rules.
I think that in some circumstances, saying "no, it didn't work" and adding something about arcane defenses maybe to give a modicum of rationale for the failure, is perfectly ok.
And I think that having the rules say that it is ok, sometimes, to do that, is great for those of us who haven't got such a good grasp of the rules and how they affect our adventures as many of those good DM's who have posted before me have.
And I'm saying this as a guy who's be DMing for 26 years. And my players enjoy me doing it, so I guess saying "no, it doesn't work" once in a while won't end the world as we know it.

/M