It is not like picking a Demigod, because the error required to pick the wrong Demigod is an order of magnitude more, and moreover, it's much easier to suck up the loss (ranked) or restart the game (custom). Picking the wrong skill is easy. Moreover, it's easy to see that 8 Demigods can be balanced overall, and not all 100 possible skills.
It only decreases variety and choice. Once I pick Hammer Smash, I MUST HAVE HAMMER SMASH. How the hell does that increase choice, because now, I can't have half Hammer Smash and half Boulder Throw. Seriously, tell me how the hell this possibly increases choice and variety? You can't choose to spec multiple abilities, which MASSIVELY reduces the number of possible combinations. Either you pick once, or you pick repeatedly. Are you aware of what the formulas are for how many combinations there are? Try throwing a dice and thinking about how many combinations, compared to throwing it repeatedly. I could choose to spec one point in every ability, or in your system, I can't. Can you give me an example of how a new spec is created, by locking out abilities?
I didn't even think about or mention respecs and they weren't relevant. The point was that your talents can be spent in anywhere you like if you satisfy the pre-reqs. You don't just pick a tree. You can have any combination you like, and I've had some weird and wonderful ones. Demigod in the current incarnation works this way too. It's like, Supreme Commander. Turtling is not a choice. That's shit. But equally, FA, where expanding is not a choice, that's shit too. The game is developed for me to play it and have fun, and that means doing with it what you like. Why the hell should you give me any choices at all? Let's just play random Demigod, movement and attack like creeps, random skills managed by an AI. Oh wait, because playing the game is defined by the player interacting with the game. The more possible interactions there are, the more variety there is in the game.
Rules exist only to enforce fair play, and create options, which means basic rules to establish how the game is played in as many different ways as possible. Any rules you make that detract from this, automatically fail. There is no reason to limit it. There is no reason to enforce rules that serve no purpose.
I still challenge you to find an RPG that forces skill lockouts in this way. I can give so many counter-examples. World of Warcraft, Warcraft 3 (inasof it's RPG elements), System Shock 2, even the failure that was BioShock, Diablo II, Dungeons and Dragons. In Diablo II, if you put a gem in a socketed item, that was it. Guess what? In WoW, they changed it so that it's not permanent. I might just take a hint.