Why? I can just play with a character for example who had full healing gear and I can stay in my tank dps gear and do the same fight fine. Or he can go half heal half damage for example and I can go full tank.
If you go too much in one, enrage timers will get you. If you do too much on the other you might not survive. You have to balance out what your play-styles are. There IS no best set.
Its all playstyles. Its all what and whom your playing with. Its all about the group.
If your group leans too much in one direction you will have a hard time. However your right, some times one direction for one character might be better, or it might be worse. Or it could be what "you think" is better and someone else might come along and prove another way is better.