By contrast. Back in Heavensward it was well renowned that MCH would do a low DPS and people would refuse to let them into parties. My MCH friend who had a really good DPS struggled to get PF clears because he kept finding groups who refused MCH's, this was before they got buffed. I also recall a normal mode Bismarck with only MCH DPS that struggled to clear the DPS check.
And there's other stigma I've seen in my time:
DRG's who floor tank, when I get a DRG, I know there's a chance I'll have to raise them
BLM's who eat AoE's to save their DPS, I know I might have to watch their health more.
3.0 DRK's who were squishy because people hadn't learned how to mitigate with it. I had to be extra vigilant versus WAR or PLD.
For those, I'd still not use 80% or even dread having any of them. If 80% of WHM's were as bad as the OP makes out, people would not be using them or would be dreading them on seeing them in their party as you say.
But WHM lacks any such level of stigma, at most there's "healers who DPS too much" and that whole row, but that's a part of their job and as I've already highlighted, this tends more to be an error in judgement and not what the OP claims, what the OP claims I find to be a small minority. Given the OP's claims, I'd expect there to be more of a stigma. And for there to such caution when seeing one in a party. I'd expect whenever I get a WHM healer that there's a high chance they're going to prioritise their DPS over their healing and when I'm a PLD be ready on hand with Clemency or when I'm a RDM ready with Vercure or when I'm an AST or SCH co-healer that I'm going to have to carry their weight.
There's the odd bad run where I might have had to use Clemency a few times because the healer struggled. The only time I can think of I've had to think about it for a whole run is when the WHM had bad connection issues not because of Glare, they were transparent about it, offered to leave as it looked to be causing problems and I told them "no, it's fine" because I was a PLD and could help with the heals and we did fine.
But the "odd" bad run is never a reflection of a large percentage of people. They'll happen. It sucks, but is the nature of a multiplayer game.