I think the reason you're unfamiliar with this is that the pace of battle in FFXI seems roughly 5x slower than FFXIV. I haven't played FFXI past level 10, but I imagine you would actually have to try to overheal because incoming damage in that game seems to be designed to be roughly equal to what can be healed at the same rate. In FFXIV, incoming damage will come in much more rapidly than FFXI, but your healing spells' amount will always be much more than the raw incoming damage overall (by roughly 2x) in normal gameplay. So in other words, HP bars are filled so dramatically fast compared to FFXI that you never need to be constantly healing for normal gameplay.

Honestly, overhealing is not a big deal and no one should ever be getting on your case about it. You really can't run out of MP from just healing unless the tank decided to pull 3 or more groups in a Level 47+ dungeon. MP management is as much of a serious deal as you want to make it, like also casting damage spells in addition to healing spells. The only stupid thing you could do is casting area healing spells when only 1-2 people could use healing.