it has its pros and cons.
with unlearn perks you can do things like give points in smithing make your uber imba dragon armor, and give the points into fighting after thats... quite stupid -.-
but of course it could be that a perk dont work like you guessed, in this case it would be very usefull.
i got this case 1 time when i learned the perk double casting healing and 10 minutes later realized i never heal with both hands xD
BUT in there e is a trick you can use

save game, try perk, if u dont like it, load agian.
same way u can test shouts.
@bashing guys: he is speaking about perks, not the level of tree, so its nonsense if u talk about leveling with something fast and then give perks it in the more powerfull!