Why would you spend all your time healing? I mean, if the game is designed that way, okay, but neither 3E nor 4E encouraged classic healbotting. 4E avoided it by using minor actions for healing spells; 3E avoided it simply by making healing a sub-optimal use of a standard action. Smart 3E clerics spend their combat rounds casting buffs and control spells and whacking enemies over the head. Unless someone is on the brink of death, the healing spells don't usually come out till the fight is over.
Both of these things are kind of debatable. I played a Cleric as my primary 3.5e character. I played in Living Greyhawk and used to fly to a couple of conventions a year to play him, so I got to play in nearly every type of party make up with random strangers...and with no house rules. I played him from level 1 through level 16.
I can tell you that when you face easy enemies, your healing isn't needed at all. Enemies died in one round of attacking from my group. Most of those battles I spent buffing or just attacking with my sword...mainly because buffs were almost completely useless most of the time. Most of them give +1 to hit...which is basically the same as delaying most of the time. All of the REAL buffs lasted hours and were cast long before the battle started. Casting control spells was a waste of spells when your fighter is going to kill the enemy before they get to fight back anyways. During these combats sleeping would have been as useful as any other action I took.
When we faced hard enemies, on the other hand, ones that we had to actually worry about, I was healing nearly every round of combat. They often went: Enemy hits our Fighter/Barbarian/Paladin/Whoever was our big tough guy. They took 80 damage out of 120 hitpoints. Then I heal them so they won't drop dead the next round of combat. Repeat. During some rounds no one would have taken enough damage to warrant healing and I might cast a buff or just attack. Most enemies had such great saves that any spell you cast on them would fail 90% of the time. Most cleric combat spells did fairly small amounts of damage compared to our Fighter so it was more valuable tactically to keep him up than to cast them.
As for 4e, the minor action thing does make you concentrate much more on the other things you can do, true. But still, the primary goal of cleric/other leaders is "putting out fires". Often that's "I use a power to give that guy a save against the stun. Then I use a heal on him so he'll survive the next round." Once again, that only really applies to hard fights. Against easy encounters, healing isn't needed and you just just do damage the same as anyone else.
It'd be simple enough for the feat to require some time to use and have usage limits that don't constrain the cleric. Call it "Healing Lore," say it takes 5 minutes to use, and you can only use it on a given subject X number of times per day.
This is certainly possible. But the problem is in combat balance. If an enemy does 20 damage in a round, then being able to heal 20 damage in a round is exactly the same as completely negating the enemy's attack. So, you can balance the game based on having that healing available...or you can balance it without that healing.
It's next to impossible to balance it no knowing if healing exists.
