My Rogue -- which I am leveling for the sole purpose of being able to open my own lockboxes rather than harassing my friends.
That's the reason I decided to finish leveling my Paladin last month, he's my BS and I wanted to open locks. This was before the 5.2 BS info came out, now I'm glad I did level him. Plus he's fun.
That's what happens with me pretty often, I start leveling an alt because
of crafting, but I keep playing it because it turns out to be fun. Like my Mage, even when it wasn't fun in the mid levels (40-60) I kept at it because I wanted to level his professions, but now that he's 90 I'm having a lot of fun with him.
However, I've found a few classes which I just plain don't enjoy, and it's their professions which drives me to keep them leveled. So if I could have everything on one character, then there's be at least two characters I wouldn't play.
Though having all professions on one character would make things easier in terms of raiding. My Mage for example, he's my Enchanter so the only way to get some of the mats to make the raid level items is to buy them or raid with my Mage... which is why I'm currently grinding up his iLvl. My main has trashed so much raid level gear that I'm actually thinking of dropping his second profession to pick up Enchanting, just so I can DE these things. So if he could keep the two he has now, plus add Enchanting... that'd be pretty sweet. ;-)