Additional levels would only be available near the end of the game whereas subclasses and races could be enjoyed through the entire game.
Probably the strongest argument for supporting these additional options, tempted as I am by the prospect of higher levels
Related topic, I hope the rumored higher ceiling than lvl 10 is true, and that BGIII is only a prelude to expansions and a follow-on game where we can get our characters into epic territory...
