Using his MP for buffing is pretty much irrelevant these day with all the auto-refresh gear existing, Siphon, Convert etc...

Speaking of usefulness, if I'm not DDing in my party then I gives them a +24% Double Attack (Ifrit's Favor), 5:30 Hastega, 20-40 dmg Enfire (even on VT mobs not particulary weak against fire) and a 3:30 Atk+9% while I can keep the role of main healer (600+ HP cure IV) Even without a form of refresh/ballad I never run out of mp (even outside Abyssea) and I don't feel useless at all for my group.

I agree that Smn need a big help for lower levels but not at HL if you take the time to equip it right, capping Skill etc...