1) Tanks and Healers are less in number than DPS so they are "more sought after" just because there are less of them available. Each job in its respective role performs just as admirably in that role as anything else.

2) It's more time efficient to focus on 1 class with occasionally side-class leveling on the side, due to the fact that you get +50% xp for any class that isn't your highest leveled one.

3) Every 5-ish levels seems like a decent rate.

4) Each craft has a REALLY GOOD skill available at some point, most of their skills are shared.