But if you can't get all the different perks with one guy then you probly can't be good at magic and fighting and thief stuff even though you could be good at everything with one guy in Oblivion.
And that's exactly what one of the biggest problems in Oblivion was. It didn't matter what you started out as, in the end one character ended up being pretty much the same as the last one you played. With only being able to take a limited number pf perks, you're going to end up with unique characters each time you play the game. You'll be forced to make real decisions about how you want your character to evolve, not just be able to spam every skill in the game.
So that's not fair so you should be able to change them later if you want to.
Why? If I decide my character is going to focus on being a fighter for the first half of the game, why should I be able to suddenly become a master Mage? You should be forced to make a decision and stick to it. If you decide you want to try something different, then you can start a new game with a new character.