- I don't have time to grind characters and skills. I could do much more interesting things like deriving the quadratic formula.
Quick access to all characters. If you get bored off of one character, you get another character to play with instantaneously that will not be terrible in PvP (well not due to a game level cap). You also don't have to grind that character all the way to the top.
Variety. Less grinding and investment in level curvature for the designers means more actual variety when one reaches the finale (melee v range v status v summoner v healer etc.). In your idea everybody would be the same at the finale, meaning the conclusion would be kinda homogeneous.