In oblivion you could master everything, everyone hated that I guess. Now your wanting to be able to be the master of everything, but not at the same time, just when it's convienant?
Maybe games should have things called "choices", and you "choose" something, and stick with it. Or you could just pick some perks for a Theif, play the theives guild, magically forget that stuff and become a Mage, then later magically become a warrior! That doesn't sound [censored] or anything!? Come on!
I've played borderlands and you can do this, but there isn't nearly that many perks or what ever they are called, and it works for that game. Skyrim isn't Borderlands, and you should play a character, that evolves naturally through your play style. Being able to switch perks=no challenge what so ever. Oh look this guy is week vs fire, but I'm a thief! Hold on....... Ta da! I'm a Mage now! Win fight, back to my thief.... for now.
Well maybe you only plan on one playthrough? You want to see everything the game has, right? Well as the T man says, "you can have anything you want, but you can't have it all", or something to that effect. This game is designed to be that way, meaning you can't do everything. If you could, then they didn't do their job. Get this idea out of your head! Or play on the pc, and I'm sure console commands can give you what you want(changing perks/skills). Oh ya, so instead of learning skills by doing you'd want to learn any skill, take it's skill points, put them into a completely different skill, then take your perks and repick them in the skill you didn't actually learn? Just so you can be anything, at anytime, for whatever reason. No thanks.