Too easy to exploit, especially for things like smithing. The Smithing Perk Tree was specifically designed to make you pick either light armor or heavy armor. Of course being that weapons sadly are included in this...but I digress. Many perk trees have branching paths that make you decide what you think is more important to you. Having the ability to reset perks would just be too easy to exploit by switching trees back and forth at your whim.

But if you mean resetting perks and not gaining any perk points...what's the point?

Sorry, but nonsense OP request. If you mismanage your play and things gets too hard for you, reduce the difficulty. Don't ruin the game.