I think people vastly underestimate the time and resources it would take to actually add an extra slot to the party structure even if they didn't go back and redo every existing piece of content, which depending on how the grouping system was structured they might have to do. Yes, you can currently have 5 players in a group but its quite possible the existing tech wont let you cap a group at 5 members and its a cold day in hell that I am willing to run a dungeon with 6 random dps and one tank every day for my roulettes.

Oh and on healing challenge..

Healing challenging mechanics is enjoyable. Healing stupid isn't. Its frustrating and annoying. More players wont add much in the way of mechanical challenge. It will just add more stupid.