You're suggestion isnt making much sense here - you claim that people dont learn how to play their job while leveling in PotD/PvP so now you want to let them level jobs instead without playing them at all? How do I learn to play dragoon when I run the dungeons on my paladin?
We're already having a passive bonus in place to help level any job thats below our highste one.
We've got that rested bonus.

I mean, I assume this is just another try of getting tanks and heals into the dungeons because you want to play your dps but cant stand the queuetimes. But no, sorry, leveling a job without playing it at all is even worse than leveling that job in PotD, PvP or with FATEs.
And unless you can only give exp to DPS-classes this can very well turn the other way too: People who dont really want to play tank/healers can now level those classes without having to set foot into a dungeon in the first place.

I also cant see how that would encourage more roulettes - the encouragment of roulettes comes from the bonus they're providing. Why would I run leveling when I got the bonus for that day? I could end up in Sastasha NM, which gives basically no exp at level 65 - even if I could pass those exp on, thats basically nothing and I'm better off doing two FATEs.

Leveling is pretty quick and easy now: weekly challenge-log, daily roulettes, daily hunts, FATEs while you're waiting for dungeons... Leveling to L70 is indeed nothing like what it used to be leveling to L50. Because before dungeon-exp-buffs and roulettes leveling to 50 was FATEs in southshroud and Coerthas.

Please, let people play teh job they're earning their exp on - even if they dont learn it in depth in PotD/PvP, its still better than having someone at level 70 telling you "Lol, first time playing this job".

Oh and also: I got 6 battleclass jobs at 70 now - a mix of tanks, heals and dps. And I wouldnt say that it was a painful grind at all (...except healing undergeared tanks through masspulls in Bardam on my scholar, but hey...) - this might sound a bit "esoteric" but I believe it comes down to your mindset here: I dont need to have everything right now. I'm okay with taking it slow, with hitting 70 on another job every 2-3 weeks. Makes it a lot less painful if you dont stare at the numbers the whole time and think "uuurgh... I'm only 63, kill me now!"
For your long dungeon-queues I'd recommend what I've been doing: Buddy up with a friend, take turns healing/tanking. One of you levels a DPS, the other a tank/healer. Once you're both 70, you pick new classes and switch.

The only place where I'd like to be able to assign exp is the weekly challenge log - would be great if I wouldnt have to worry about completed-dungeons-exp going to waste because I threw an expert roulette in.