chitzk0i
Explorer
What would be a reaonable premium? I.e, how much should you buff them to make them viable?
First of all, Majestic Word is only a minor action. Using that doesn't take up much of your turn, so making a healing ability use an immediate action isn't a substantial benefit unless it's an emergency "ally drops to 0 hp" power.
Furthermore, the power is kind of unhelpful. These guys just got missed by an attack. Are they really the ones you need to heal?
Lastly, it's not much healing. At level 6, half level is 3 and your Int is probably 3 or 4. Six or seven hit points healed? Once a day? Not really impressive. Could be useful if you could affect most of the party with it, but most area attacks are versus non-AC defenses, which are pretty easy for monsters to hit.
This power may get nicer as you get into paragon, though. Once you have a few more utility powers to choose from, you may be willing to use a slot for a corner-case power that's cool and flavorful but doesn't come up much. Plus, as you level up, I think monsters get more (and bigger) area attacks, so you'd get more chances to use it.