Taking a look at it, it's a Cleric 27, so only available to characters well on their way to deityhood. And there's no reason they couldn't use it every 5 minutes.
It would heal somewhere in the 20-ish neighborhood, per person, every 5 minutes. If the battle was long and tough, they could probably manage to heal up fully in a half-hour to an hour.
I guess I don't see why this should bother me.
Yes, it sidesteps healing surges, and it's infinite between-encounter healing. However, if the party can spend an hour waiting for the cleric to burst every five minutes, they can probably spend 6 hours on an extended rest just as easily. What's more, they don't get any healing surges back, which would prove very useful in combat.
Other than hitting a milestone easier if your DM is a pushover, what's the flaw here?
-O