In my games, if a party cannot provide healing for itself, they reap what they sow. I don't "tack on" an NPC cleric if they don't have a cleric. Other groups will vary, but in my experience, 3E has provided such that a cleric's healing is no longer the only way to provide support. Keep in mind that not ONLY can one-third of the party cast healing magic now (Bards, Druids, Clerics & Rangers), but healing potions, scrolls, and items are suggested to be affordable, AND a party of adventurers heals MUCH faster than they once did.
In the "olden days," if you had only one healer, and your group was severly beaten, you STILL took weeks to rest and recuperate, because the absolute most you regained per day was 1 hit point, or 2 or 3 with complete bed rest and healers attending. A 70 hit point fighter could take a month to completely heal without help. Nowadays, you're looking at MOST two weeks for any character to heal hit point damage from zero, whether they be 1st or 20th. (Amusing thing - a high CON score means it takes LONGER to heal from zero!)
A party can, quite well, live without a healer in 3E, but they need to play with a modicum of caution. In 1E or 2E games, more than a modicum of caution was needed - it was required to be crafty as Erwin Rommel or Francis Marion.
