See, to me, the plane of origin doesn't dictate what I need. Faerie, Shadow, Gehenna...to the podunk villagers IMCs, they're all just Far Away Places. Heck, the next valley is probably Far Away, too, so whenever a traveling merchant comes to town it may as well be an ugly bloated thing from a faraway dimension.
Storylines help dictate what I need. And I need a diversity of stories. Which means a diversity of villains with a diversity of agendas. Gehreleths are interested in taking and keeping prisoners. Yugoloths are interested in selling their services and spreading their shadow agenda. This is distinct from the Devil's agenda of corruption or the Demon's agenda of destruction. It's also different from the Faerie motives of flighty personal desires, the Slaadi's agenda of spreading rampant chaos, and the Modron's agenda of establishing peaceful order.
Those are all different stories, stories I'm interested in telling in my games, stories that will not be served by smashing everything vaguely menacing and otherworldly into the same pot.