It’s true that the raw concept of over capping isn’t inherently a bad thing, such as with a cooldown like Rhizomata in your example, but both Aetherflow and Addersgall are tied to MP management, which to be fair, is not a prominent concern for healers, but forgetting or not using these resources does cause you to loose MP restored over time.

The problem with this is you don’t know if something bad will happen and you need to throw out raises to try and salvage a botched run. This isn’t a common occurrence either, but the best thing to do is to ensure you’re getting the most MP out of your kit so that you’re as prepared to handle circumstances like that, and pick up the habit of doing that always so that you’re not forgetting to do it when it matters.

If Aetherflow and Addersgall had no connection to MP management, or anything other than specific resource management, it’d be perfectly fine to let sit at cap. It wouldn’t necessarily be a beacon of exciting gameplay either but it wouldn’t feel bad to not have a spender of sorts.

Overhealing also doesn’t actually have a negative impact, so your argument makes sense. If it doesn’t really matter, what’s the problem? Well, from my perspective, it just doesn’t feel good or make me happy as a gamer to know that sometimes I’m just going to burn heals I don’t need. There’s a lot of players who want to feel that the way they’re using their tools is as tactically as possible—that nothing that’s used is going to waste, so to speak. So when you’re wasting healing just to get some MP back, it feels sloppy and defeating—that there was no way for me to make use of both sides of that action and that translates to a negative moment in my gameplay.

Not everyone feels this way, but I’d argue it’s a fairly common way to feel for a lot of gamers. In Persona games, for example, I don’t want to accept someone’s invitation to hang out if I’m already at the point of being able to advance my relationship with that character. It’d feel like wasting the points I earned because they don’t cross over to the next rank in that social link. It’s the same mentality.