I will agree, what prevented healer DPS in XI was the preciousness of MP. It's a bit less of a problem now a days thanks to having things like 10-16mp/tick refresh available via gear and songs but it's still important not to waste MP on frivolous things.

Also healer nukes were generally very weak. WHM nukes were both expensive and had long recasts so you couldn't just spam DPS spells all day. RDM nukes capped at tier... 3 I think at 75? Which were pitiful in damage compared to BLM or SCH. And SCH nuking required the use of a Stratagem (think Aetherflow in some fashion) to get access to their largest nukes, and those Stratagems could be used to augment your healing spells.

The funny thing about healer DPS in XI is the fact that meleeing as a healer would be more effective than nuking as a healer - especially since WHM can essentially get infinite MP with the right gearing and party setups and some of the fight benefited having your WHM be afflicted with debuffs as well due to Esuna.