So- whilst most spell skills do reduce magicka usage (or enchant usage) the perks Do most of the reduction. Things that increase the act by a precentage seem to do most of the power changing too.
Then there's the crafting skills
with smithing you improve what you have (fine weapons, legendary weapons etc) but the Armour part is a bit silly. Why do i need to get perks to make new armour? Surely skill alone should be enough to make new armour
With enchant.. weapons might change with skill but i for one didnt notice any difference in the quality of enchantments between starting levels and 53 (before i got it's first perk)
and with alchemy- if there is a difference then i have not noticed it.
Problem is that some of the perks are just... Not neccesary. Some of them should automatically be unlocked in my opinion (n/j/a/e/m in magics or 0/25/50/75/100 in one place perks and the smithing perks too whilst we are at it) il probably make a mod with this. But still... a lot of things done with perks should have been done with skills.