I see what you're saying, but you're trying to solve a problem that isnt there.
If a player diffuses his points across all skills, all of those skills will be negligible anyway. So really, people already focus their points into a few branches. For instance, as Torchbearer I roll all my points into Circle of Flame first, Fireball second, and attribute upgrades third. Once those are maxed out, I can have a little more fun exploring, maybe find something that works better.
Infact, I believe your suggestion would cripple the game. As it is, a player can explore the entire tree of his Demigod in a single game, assuming he gets to 25. This allows him to see what he likes, and see whats effective, and what can be used in tandem. Your level cap would result in people sticking to the same choices in match after match, and would result in anyone who tried something new being berated by their team mates for 'not using what works.'
As an added bonus, a level 25 game is majestic, all skills are being used to their full potention. The Rook raises towers, then absorbes them, rolls boulders and hammer-slams while his trebuchet, archers, and tower of light all fire into the masses of creeps. The Torch bearer is raining ice, throwing fireballs, freezing, burning, and exploding everything around him.
This is important because a level 25 game has already gone on long enough, and it needs to be interesting and engaging.