What if you come to a pack where your cookie cutter isn't working out, and it would be ten times easier for you to swap a couple of your skills and passives out that would mow that pack down? Or you get to the future boss and realize your cookie cutter is just not viable against him and you will definitely have to re-think your build?
The rares elites still generate random buffs game to game which may completely eat your cookie cutter spec alive.
There is too much comparison to D2 going on where you can just pick the one best spec that will get you through the entire game. The general mentality seems to be following that same degree of laziness so they never have to open their skill tab ever again and just spam mouse clicks and hot keys without any kind of strategy involved.
I don't know what the other quests and acts hold, but I am pretty confident you are going to be opening that skill tree to make adjustments OFTEN. Worry less about what the "best spec" is going to be because in my opinion, the best spec is going to be the best spec you can puzzle-piece together in the moment when you hit a snag and need to adjust. That's just one of the things that is going to make this game awesome.