Is it really a good idea to have a blanket rule? Teams, prospects, players...every situation is unique and should be handled as such. I'd say there is about a 1% chance that any mid-level prospect would be able to beat out Grabovski for a center position in a fair competition. How is it advantageous to create a roster where young kids, short of phenoms, simply cannot break in? That doesn't help build a team for the long haul.
It is never a good idea to have a blanket rule for anything. That said, it's not like adding Gabrovski gives the Sabres four stud centers that no one could ever hope to unseat. The way I see Gabrovski (for example) fitting in would be as a player who would step onto the second line, and push Hodgson for the first line job. Now Grigorenko, Girgensons and Larrson potentially have to fight for two more center spots. Excellent! If Girgensons is not able to secure a spot here, AHL it is until he can knock someone else out of the lineup. Same for Grigorenko, he should be looking up at a player like Gabrovski and know he has to become better than him to ever get to the second line, let alone the first. If he is a lazy, moody weirdo like some people here fear, he won't be able to grab that spot. In fact, there should be other prospects breathing down his neck for his spot on the roster and kicking him out of the lineup (or out of a center spot at least). And so on and so on.
In reality, it's unlikely that all of the prospects we have at center will become NHL regulars, so make them show who will be able to fight for that opportunity.