1. I am a 1.0 vet. I have literally since my first footsteps in Eorzea had a CNJ (back then CON) stick in my hand. I liked the class. Back then it was hybrid Nuker/healer. When it turned into the WHM, I stayed CNJ/WHM as my main, even though I had THM 50 as well, because I liked it's lore/role and just basic gameplay better. There was no 'faster queue' consideration or even 'I'm the healer I'm needed' consideration. I picked what I liked and what worked with my friends. I am leveling PLD currently because it's in short supply in my FC...and because it's my last job to level. Again, not a 'faster queue' consideration.
2. SE already does have incentives for being 'a player in need' <---SE's words not mine. Every day in the Duty Roulette whichever role is most needed gets a small bonus to exp/gil/tomes/seals (depending on duty). 85% of the time, that bonus=tank bonus. Now, maybe those need to be beefed up to be more enticing, but the program already exists to reward those willing to take on needed roles. But no, outside the completion bonus/player in need bonus, there shouldn't be an additional reward for simply showing up with a sword and shield. At some point, they need to be doing the dungeons because they want to/want the actual rewards from the dungeon.
3. If content isn't being played...or if specific party roles keep avoiding the same content, then SE needs to look at why is this happening and adjust the content/party role so it IS being used. For example SCH/SMN's use to avoid Aurum Veil due to the difficulty of keeping the pet alive and still healing party/dding during 2 of the 3 bosses. SE fixed that dungeon that those boss room poison effects now don't effect the pet. Pharos Sirius is avoided currently by many, but especially WHM's. SE may or may not change how Siren's Charm effects work...or their timings to encourage more whm's in PS. Warrior had big adjustments in this patch because people were choosing to double up on PLDs rather than do content PLD/WAR. The list goes on. So sometimes it IS good to let the chips fall where they may and not force everything. If people don't do content, play a certain job, there is a reason. It is SE's job to find the reason and solve it.
But the whole 'there's not enough tanks...so if we make it so they can't keep abusing us there will be even less tanks' argument, is faulty at best. The way join in progress can be spammed needs to be changed. If by changing it it now means tanks will refuse to queue for content, then SE will have to solve that. ...but really, tanks won't stop queuing. They believe it or not, need the items too. They are just taking advantage of something while they can. When that option is taken away, they will maybe whine about it for a bit, and then come back in and play with everyone else.