Even if everyone played a healer and tank it is still unbalanced. There are not enough healers and tanks to level up compared to dps. There are 9 dps or 8 with smn/sch. Compared to only 3 tanks and healers each. After people level up their healer or tank they move on to the multiple dps rolls. That's where the imbalance comes in. People spend more time on dps because there are so many jobs to level.

I already level two healers and I'm working on my last one. After I hit 70 I'm hanging up my healing hat for a while because I don't have another healer to level and I'm going to focus on leveling up my dps. This means there is one less healer for the weeks/months it takes to me level up 5-6 other jobs. If there were more healer jobs I would level it and this would increase the healer population. Not everyone is like this but I'm pretty sure that plays a major role in it too. So you would technically have more people playing healers/tanks if there were more jobs.