When you're a Dreamstate Druid, you're just another direct healer. Your raid probably already has another 5-7 of those. You can heal, and sure you can heal just fine, but there's no reason to bring a Dreamstate Druid over anyone else at that point. You're not going to put out better healing, you're not going to sell yourself or set yourself apart.

However, as a tree properly rolling HoTs you DO bring something else to the table that no one else does. The important part about 25 man raid healing is engaging all of your healers together to work as a team. Everyone plays to their strengths, and it goes much more smoothly.

When my tree ass is not in a raid for some reason, while the HoTs may not seem like a big deal to me, their loss is felt handily by the other healers. The buffer for tank HP they offer, as well as being extremely responsive to damage taken is invaluable.

Dreamstate Healing, Moonkin tanking, Shaman tanking... all of these things CAN be done, but they're not optimal and when it comes to selling yourself as a productive member of a 25-man raid, they're not likely to be what people seek out.