This was my FFXI experience.

I started as a WHM, because I wanted to heal. I played, for what felt like a long time, and finally got to 20. My friend, who was the reason I picked up the game, helped me do the quest you needed to pick a second job. Then he tells me that I need to pick BLM (I think that's what he said) so I changed, and started leveling that, and made it about 2 levels before I went "Wait.... so... basically I am expected to always keep this sub-job leveled up as I level up my 'main' job... so I am FORCED into leveling 2 at the same time, because being a lvl 40 mage without a subjob at 20 would make me a slacker." and I logged out and never logged back in.

It wasn't so much that I disliked the idea of subjobs and how that whole thing worked, it's just that you couldn't be a "real" WHM without also spending an assload of time leveling up a subjob that, frankly, I didn't give a crap about at the time. It felt like a really artificial way to keep me playing.