If the game's fiction (the gameworld/setting, whatever you want to call it) is important, yet a player can use Come and Get It at any time - even it if makes no sense in the game's fiction - yep, I see it, the game rule trumps the fiction. That is a similar occurance to what happens in board games.
More importantly (to me), it reduces player choice to tactical manipulation of the rules on the battlemat, and not how it relates to the gameworld's fiction. Smart play is "chess play" and there is little sense in manipulating - or even caring about - the fiction. Even more so if all challenges are level-appropriate!
If one grants the DM the authority to overrule Come and Get It in order to maintain consistency in the fiction: "No, those archers are not going to jump off the wall to face you" (a possible result of Come and Get It), then what is the point to having a suite of powers pre-defined in the books?
But yet something about 4E intrigues me, especially since I consider the DM to have the authority to overrule powers (of PCs, NPCs, and monsters) so that he can maintain the consistency of the fiction - which is a primary responsibility for him. (In my case, that responsibility is there so that players can make smart choices.)
This discussion (and the short one I had with Ariosto on skill challenges) has been interesting. I need to think about it some more.