Basically, for fire resist, if you don’t get a total resist, you get the interrupt. At *max* resist, you have a 25% chance to resist the spell and a 25% chance to resist all damage. At something more reasonable, like 200 resist unbuffed, it’s more like a 12% chance to resist and 12% to resist 100% damage. Concentration aura gives 35; more with talents. The healer taking a little extra damage is alot less likely to wipe the raid than the healer’s spell not getting cast in time.