In a comment on the post about minimum scores needed to switch classes, JB wrote
... why would a person who's put years into a particular occupation/adventuring style suddenly "switch?" If he'd had the stuff to make it as a wizard, and he didn't do it when younger (because he was too lazy or whatnot), why would he suddenly do so at the age of 35?Because tastes change?
I realize the 20th and 21st centuries aren't the same as medieval, pseudo-medieval, ancient, or heroic times and that people didn't routinely change careers. In fact, I've already come out against treating classes as profession, so the idea of changing your profession is not something I want to incorporate into a fantasy game. But even when talking about broad archetypes, there have always been people who changed over time. Gilles de Rais, a knight who served with Joan of Arc, later started doing theater, of all things, and failed miserably at it, losing most of his enormous wealth. Then, he started dabbling in the occult, looking for a way to find money using magic, which led him to killing young children and eventually was executed.
That's an extreme case, but there are plenty of historical and literary figures who are described as having started as one thing and switched to another later in life. In pure game terms, building up a bunch of hit dice as a Fighter before switching to Magic-User gives you some survivability and opens the way to casting spells while wearing magic armor. The default way of implementing a Fighter with a little bit of magic or a Magic-User with a little bit of combat skill is class switching. But aside from that, maybe a player is only a little interested in one class and only wants to play it for a few levels, then switch.
Why forbid it? There's certainly no realistic or literary emulation reason for doing so.