There is "a build" because you will lose the buff "Nephalems Valor" when you decide to swap your skills on the fly while playing. This will motivate the players to build one build. But they can easily change it between the runs.
Where is the difference to let you pay gold for a rebuild like in WoW, to letting you change your skills everytime (but punish you a little for doing so).
Imho you are just whining without thinking of it.

must-have-list for platforms i don't own yet:
WiiU: Donkey Kong
XBone: Dead Rising 3, Ryse







