I play as all three classes in the crucible. The reason why the titan isnt played that much, in my opinion, is because people see the titan and want to take him out quickly as possible due to obvious reasons. The hunter is basically a sniper so they hang back. The warlock to me seems more like fire/artillery support due to their nova bomb power so he is the next target to go for. But i also think titans arent played very much because their also slow to move around with.