A person with higher max MP can also know how to manage MP. But things can go wrong very fast and being able to deal with it and salvage a run rather than chalking it up as a loss is something I consider pretty important.
I've always looked at MP as a buffer. The larger your buffer, the more room your entire team has for error. There are situations where people will not dodge eruption, where Ifrit will never jump and give you time to regen MP, or where people will simply D/C. That extra MP often meant the difference in being able to raise someone while healing or just sitting there unable to do anything. The extra MND or healing potency has to give a significant enough boost to my role for me to want it over extra MP.
That aside, I saw the tests Kaeko posted yesterday and healing potency does seem worthwhile to slot. If that rough draft of the cure formula is correct, healing potency has a huge advantage over MND and provides enough of a boost to make it more efficient than MP in a MP:HP healed ratio.
MND and MP are dirt cheap though. Personally, I sold all the healer's hand materia I ever got.