Okay, group project time. Because some of you are better with numbers than me and because there's some weird stuff some of you might have a far better idea of how to calculate.
Tank/DPS and Healer cure potencies may not all be the same (in fact, I believe everything I've read and seen says they are not), but given a discussion got me thinking: How much healing would be needed in a 4 man encounter to make Healers "necessary".
Here I'm going to try and quantify this concept. For the sake of argument, "necessary" at its LOWEST should be defined something like this: Damage to the party that exceeds non-Healer party configuration maximum possible healing; damage to Tank that exceeds said healing + Tank self-sustain modified by mitigation.
In other words, where if you stack a 1T/3DPS party with maximum healing and the DPS use their heals on CD, the party collectively is taking more damage than they can keep up with, and likewise, that if the Tanks are doing their rotation perfectly, they are taking at least 1 HP damage MORE than that such that they are slowly losing health over the course of the fight without outside healing.
If this was the level of incoming damage for a 4 man, then it's the level that would necessitate a Healer be present in order to output greater healing than this amount of incoming damage.
Make sense? Clear as mud? Alrighty, let's begin by trying to quantify how much healing non-Healer Jobs can produce, then we'll pick the highest Tank and three highest DPSers (or the highest one 3 times?) and see what that total ends up being. Fair starting point?
EDIT: Oh, right!
I'm normalizing to 2 minutes (we're talking 4 mans here, though this COULD be extrapolated to 8 man encounters as well...for 8 man normal content, anyway), and for the sake of this argument, consider single target boss fights. We can also do data for trash packs, but those are more variable, so we'd have to pick something like 8 targets (for the Bloodbath AOE type stuff), though I think it's fair to say Bloodbath/Bloodwhetting are just going to generate a fullcure for their duration in high AOE situations anyway, whatever potency that would evaluate to.
Also, 500 Potency for a Healer in BIS-ish gear (630 + upgraded Mander weapon) is ~7-8k healing off a Cure 1, as a base reference point. 600 potency would be somewhere around 10k.