
I think it's perfectly acceptable to level classes "OOCly" that your character would never touch ICly in order to facilitate gameplay. Say, hypothetically, that we reach a point where having a particular cross-class skill is considered "mandatory" (or at least highly recommended) in order to be successful at your chosen job in end-game content. If you want to do that content and perform your job well in terms of game mechanics, you're going to need to level up that class to get that skill. If that class is something that your character would never do, then you're at an impasse, and you either need to decide whether to give priority to the game mechanics, or your RP.
While the OP has a good point about how a character who's skilled in one discipline may easily be able to justify being skilled in another similar discipline, I'd have a harder time ICly justifying something like a career white mage suddenly training up to become a skilled pugilist, without some serious IC training time. Or, say, you have a character who has an active dislike for a particular type of combat, and would never do such a thing ICly. While it's certainly possible to justify multi-class proficiency ICly, I don't think it always makes sense.
Personally, while RP is fun, this is an MMORPG for me first and foremost, and I have no problem with doing omitting purely gameplay-related things from my character's IC history, and I will never let IC logic stop me from participating in a piece of game content that I want to participate in. If you can make it fit ICly, then that's even better, but that's not something that I would ever consider a requirement in order to do the content. I'm not going to ever sit out of a fun raid because "my character would never do that ICly," but that's because I always like to participate in endgame content in MMOs and I'm not at all a career RPer (this is the first MMO I've ever RP'ed in, and may possibly be the only one). People who come to MMOs purely for RP and couldn't care less about the game mechanics will undoubtedly have the complete opposite view.
This is purely a matter of opinion, of course, and people who disagree with me are perfectly entitled to their own opinions. But the OP expressly asked for an explanation from people who are willing to level whole classes purely OOCly, so there you are.
While the OP has a good point about how a character who's skilled in one discipline may easily be able to justify being skilled in another similar discipline, I'd have a harder time ICly justifying something like a career white mage suddenly training up to become a skilled pugilist, without some serious IC training time. Or, say, you have a character who has an active dislike for a particular type of combat, and would never do such a thing ICly. While it's certainly possible to justify multi-class proficiency ICly, I don't think it always makes sense.
Personally, while RP is fun, this is an MMORPG for me first and foremost, and I have no problem with doing omitting purely gameplay-related things from my character's IC history, and I will never let IC logic stop me from participating in a piece of game content that I want to participate in. If you can make it fit ICly, then that's even better, but that's not something that I would ever consider a requirement in order to do the content. I'm not going to ever sit out of a fun raid because "my character would never do that ICly," but that's because I always like to participate in endgame content in MMOs and I'm not at all a career RPer (this is the first MMO I've ever RP'ed in, and may possibly be the only one). People who come to MMOs purely for RP and couldn't care less about the game mechanics will undoubtedly have the complete opposite view.
This is purely a matter of opinion, of course, and people who disagree with me are perfectly entitled to their own opinions. But the OP expressly asked for an explanation from people who are willing to level whole classes purely OOCly, so there you are.

Adelpha Sarantas (ARR) / Aldrisi Shirantas (1.0)
Currently retired from RP.
Currently retired from RP.