They have to have a dedicated healing profession. 1 dedicated healing slot per character is not enough to clear a dungeon. I'm thinking that the one slot for healing is to help with the persistent world.
See, that's the problem with as to why the 'holy trinity of mmos' are so prevalent. It's all about design. I contend that a boss doesn't have to have 15 Million hitpoints (I don't even know what the raid bosses in WoW have now, but it is waaaay above that) and a few devastating attacks and phases in order to be challenging for a raid.
It's all about design. Left 4 Dead is a prime example of this. No healer class, and the only way you'll get health is through medpacks and pills. Compared to that, having a slot that provides healing forever could almost be considered overkill. Guild Wars didn't have a z-axis, so people couldn't swim or jump. It was a noticeable thing, but not a game breaker... because people didn't
need it. It wasn't a part of the original design. Same goes for crafting. Crafting is an almost essential aspect of an MMO, yet GW doesn't have anything that can compare with it -and nobody misses it.
My hope is that there will be no healer class in GW2. By doing so, it will finally be able to cast off the trappings that have been bogging down the genre for so long. Instead of thinking of doing things the way they have been traditionally done with a healing class, think of all the things they would be able to do
without one. By the elimination of but one of the pillars of the trinity the whole game dynamic by necessity changes, and in my opinion it can only change for the better.
Sorry, I get very worked up every time I even think about the Tank/DPS/Healer triad of MMORPGS, so I'm going to stop there...