The other extreme, that a character never gets the chance to step out of his chosen profession, is equally in keeping with a CRPG that keeps one field open for "class" (which can never be changed) and another for "level".
In fact, between the two extremes, I find the idea of characters picking up class levels in a "random willy-nilly" fashion to be more in line with the idea of characters as organic beings that go through changes in interests, philosophies and goals (or that just do whatever seems to be the most convenient or advantageous to them at the time). In the extreme case where characters could only have a single class, it would be difficult, if not impossible to portray a character who was raised by a thieves' guild and, after successfully pulling off a string of burglaries, manages to acquire enough money to retire from crime and turn his keen intellect to the study of the magic. Or a mercenary captain who has fought and killed all his life, and has become sick of the bloodshed, and thus decides to become an acolyte of the goddess of peace. Or a wandering minstrel whose closest friend was killed by a ghast, and decides to dedicate the rest of her life to destroying the undead.
And if, like me, you just enjoy tinkering with combinations of classes to see what synergizes well with what, or you like coming up with fanciful explanations for how your character happened to become a ninja/druid/warmage, don't let anyone tell you that you're playing the game wrong. If that someone happens to be your DM, you owe it to yourself to find another one.