It's a weakness penalty because that 15% additional defence becomes a baseline, and when you die, the healer isn't likely to stop and recast protect until the fight is over, if at all.
Speak for yourself. I'm one of few who keep it on my bar (no swap-macros, no farting around) and apply it after raises. We're in a point in the game where so few cross-skills are even remotely useful (never-mind mandatory) that you can get away with it.

Not saying that fixes anything - just that not all Healers are as described.