What you described is exactly how I interpreted "for RP reasons". I actually don't see any other way to define it. While there would be a tie between a character's skills and his identity, a PC who changed "profession" may not change class, while one who changes class may not see themselves as changing professions. The Conan was a "thief", but that doesn't make him a rogue. And the bard who "finds god" and takes up the cloth isn't automatically going to change his spell-casting regiment.Neither maximizing nor role-playing reasons, in my case. I usually multiclass to achieve a concept I'm not managing to pull with a single classed character. For example, I wanted to create a Raiden-like character for a friend's game, and I used a multiclass monk/tempest cleric to achieve that.
In fact, must players I know multiclass for similar reasons. The fighter/mage build is probably the poster child for this phenom.