I have one heat, and I was concerned going in that to have only one plugin instance of overbridge would be confining. When you buy a plugin, you typically want to know the economy of it in terms of CPU budget and what it would cost you out of pocket to make two or more instances of it in hardware.
So far, either one iso or one submix (or just the instruments and not the vocals) has found the right amount of heat.
Sometimes you don’t want chipotle or smoked paprika or more char on everything. Sometimes it’s simply the wrong flavor. I do get your point about putting your mix through some actual circuitry.
And, yes, if you used it as a channel strip, you might want more than one. I’m tempted myself.