So, imagine the following.
New player joins the game, tries a character, isn't sure what each class does and randomly picks 1.
Turns out it's a class/gamestyle he does not enjoy, so he wants to roll another character.
Now, since he's just trying the game out, he's stuck with his initial choice unless he spends money on the game.
Which, he's not likely to do just yet.
I believe it'd be good for the growth of the game to allow 2-3 character slots for everyone, supporter and non supporter so people have more liberty to experiment with the classes before sticking with one.
Currently the "1 char per account unless you support" is restrictive in a way that will limit the game's population growth.