Probably it's rare because it's a big powerful power to have? A channeler could, given appropriately careful packing of subjects, cure more people in a minute than the most powerful non-channeling cleric could cure in a day, though only of light injuries at Merrin's present level.
The cleric of Asmodeus has no idea why Erecura would do that! Except that Erecura is too mysterious, and even the gods too unable to forecast the future, for it to be anything as straightforward as 'there'll be some big accident at the Palace where Merrin has to heal everybody'. Probably.
Anyways, she could grab a dagger from Security, give Merrin a very shallow cut that would heal naturally, stab herself a bit deeper than that, and they could try the thing!