Yeah, because time is the important metric here.
DPS stands for “Damage Per Second”. The higher your DPS, the faster things dies. The faster things die, the easier the fights are (usually).
The TLDR of it is “wasted heals = wasted resources”. Not unlike misusing GCD’s as a DPS (like in your example of using corruption or agony on an extremely low HP target instead of drain soul).
How much overhealing is bad is more subjective. It’s not something so simple as “if you overheal even 1 HP you’re a scrub healer”.
Overhealing naturally happens.
Sometimes doing stuff that leads to overhealing (like pre-hotting or chucking down a big AoE heal like Ashen Hollow) is the correct play but will lead to bulk overhealing.
Sometimes you just mispredict and you heal the Blood DK immediately after he’s Death Striked.
Sometimes you and your co-healer aren’t on the same wavelength and you both top up the same guy.
If you’re looking to perfect your gameplay as a healer, you’d look at overhealing because it gives you an insight into where you chucked out heals that simply were not needed. But you gotta have the context to figure out if your overhealing was necessary or not.
If we’re sticking to the original comment, DPS should be looking after their own survivability by using defensives and things like healthstones and health pots.
Maximizing DPS helps everyone in the group. The faster stuff dies, the easier content is. A healer should chuck out damage when they can, but it’s never their highest priority.
If a DPS can use a healthstone instead of needing to be topped up by the healer, then that saves the healer a GCD or two that they can use to flame shock or lightning bolt something instead.