I can see your point about single jobs. And I'm pretty sure you know the myriad of reasons why it doesn't work too well in mmorpg's so I'll skip that. The point I wanted to make is, you think Vivi when you think of blm because he was a blm. You think Tifa when you think mnk because tifa was a monk. Let me ask this question. Who do you think of when you think Dragoon? Kain? Why not Cid Highwind? Or Mog from FF6/3 (yes, mog danced but he was the only character to start with a polearm in ff6/3)? Or Ward from FF8? I think the association is kind of reversed here. FF6/3 was a game where most of the characters didn't have set jobs (short of gogo, gau and umaro). Locke was a thief and Blitz was a mnk, but we don't really think of them as such. Even in FF7, anyone could steal, anyone can cast whm/blm/smn/blue/etc magic. But Vivi and Tifa didn't stick in our minds because they are a said job. They stuck in our minds because of who they are. Like Kain. A lot of people want to be dragoon to be like Kain. Not be like Kain and hence a dragoon. And, I personally, think that above it all, that's the most import thing. Not the job you, I, or they are, but the person we all are.
And yeah, a small part of me kind of wishes that you can only level one job. It would make people focus on being great at one thing rather than picking up a job and dropping it like one would with gum. But in the end, what's at fault isn't the system. It's not the system that made people treat jobs this way, it's the way we play and the way people treat the game. Like with To Kill A Raven. Guides and people kept telling me that you NEED blms for the meteors. Nothing else works. Well, our archer/bard was 1 shotting meteors. If we treat this game more like an exploration and less like "get ALL OF THE ACHIEVEMENTS!", we'll get more of what you talked about and less of this throw-away mentality.