I actually think awarding EXP for the "adventurer in need" bonus could be counterproductive - at least on the assumption that the majority of players will level all classes. Constant bonuses for tanks and healers means that they need less roulettes than a DPS class to gain the same amount of EXP.

Additionally, there are proportionately more DPS classes than there are tanks and healers - ie. if you had one player of each class and divide them up into parties of tank/healer/2xDPS, you'll have DPSes left over.

In short, the system is set up to require you to do more leveling roulettes as DPS than as tank and healer combined, even before you approach the fact that tanks level faster AND less people want to play them.