One of the groups I'm running spent a large portion of heroic tier without a leader due to a bit of a revolving door of players. The swordmage was able to do a decent amount of damage reduction/prevention, and then the group also picked up a paladin who specialized in healing. As the group was about to hit paragon, a new player came in with a Sentinel druid.
All in all we currently have 6 players running with a single Sentinel and paladin as the healers. The swordmage helps some but does more damage prevention than actual healing. A seventh player will likely be back from his thesis-induced break and is running an ardent. All in all, even without the ardent, the group has a ton of healing available and even with MM3 damage numbers, I have a hard time truly threatening the party with death -- the closest call coming when the ranger decided to go off and investigate the hostile tower on her own, leaving the rest of the party a couple floors behind.
That being said, before the party gained much in the way of leaders, I did implement one house rule. That being that a healing potion allowed you to regain a full surge of HP. I will say though that if you are going to run a largely glass cannon party, you better make sure you hit early and often though. Odds are the dice will eventually catch up to you.