If they just give more stuff based on how many tanks (or other roles) are needed, it's then able to be abused, as tanks become more likely to wait longer to join a roulette until the reward is up to a level that they actually want.
It would be better to add the Adventurer In Need bonus to every dungeon, raid, and trial, as well as straight up increasing the roulette reward. Rather than doing one dungeon per day per roulette and only getting the reward once, the tank is then free to do multiple runs to help out in multiple dungeons. On top of that, more dungeons are more likely to have Healer In Need and DPS In Need rewards to help out with new tanks that are trying to level up.