The problems I've personally seen, and sometimes faced, are not things that can be easily remedied with incentives. The two biggest things that come to mind is the social aspect to it and the mechanics at early game compared to late game. On the social side, a good chunk of players are extremely crazed about perfection of the roles. So if they run into a newbie or someone not in current top gear, some already become agitated before the dungeon even begins - and sometimes outright leave. A personal example, I've just come back from about a two month break, with my PLD a couple months behind that in gear. Technically its viable to play in the roulette's, but its on the lower end of the iLvl and I couldn't keep up with the pace most people want. Another problem along this point is the difference in enmity gain between iLvls, it doesn't foster the idea for people to go outside their norm. So given all that, I stick to my DPS as its "easier" to not show as much for not having top current gear.

The mechanics side of things, tanks are really hard to get into at early levels. Just yesterday I was toying with my alts PLD, in full HQ/Dungeon Green gear for current level, and still was being outdone by the DPS in long-winded fights (the only reprieve from this for boss fights are mechanics that force the DPS to break off attacks). This eases up a lot when the tank jobs get their enmity boost at 30/40 but still doesn't leave a good first impression. Healers are a bit of reverse in their role of mechanic changes. It starts off rather easy, then progressively becomes harder to the point where I've seen some people just stop playing them. By late game healers are expected to keep everyone fully healed, Cleric Dance for maximum dps, make up for anyone's mistakes with extra healing/status removal/revive, and sometimes know when/how to tank/kite tank if a situation is going bad or calls for it. Some gamers can handle this easily, others can but rather not deal with, while some just can't.

In the end I want to say I'm all for making incentives to play certain parts of the game, but in some aspects no amount of incentives will change some peoples desire for a class/job.