At some point, the game warns you that chosing a class will prevent you from chosing any of the other classes... Except that when that happens, you probably won't know what other classes exist, and even if you wait longer, you'll probably never know if you know of all possible options. Basically, the game wants you to not blame it for not giving you enough information to make sensible choices.

I think I'll go back to "increlution" instead.