I would think that any blu already knows that even with the scaling of the cure spells and buffs we have it doesn't come close to making the job a suitable main healer replacement, but it also doesn't mean they absolutely shouldn't cure/try to help if the situation calls for it. By that I mean instances where temp items aren't raining from the sky, you're not in abyssea with an endless supply of mp for whm, instance battles such as legion or "shit hits the fan" fight. I have had at least the common sense to help with curing in those situations, but as stated it was limited to only those in my PT.
The way the OP said what they did, and the consensus I'm gleaning from additional responses, is more so "What is the reason for not allowing it/limiting it" not "I want to be main healer and dammit SE won't let me cause all my shit is self cast or party cast only."
Only reason why I even mentioned /rdm is because it is used for cleaving. I would think anyone with half a brain would use /rdm for cleaving also given the traits/buffs it gives to make cleaving easier. Didn't bring it up cause I fill in as a healer or have some delusion that I am a healer with it.