Canonically, there wasn't much "miscellaneous" about Infernals. But the fanbase was hungry for such a thing, and very ready to hammer a round peg into a square hole.
Exigents, meanwhile, are a square peg.
Still, Infernals have their own niche and fill it well. I don't see any reason to do away with them that doesn't apply just as strongly to Abyssals.
Actually, the only splat that I couldn't ever see cutting is the Dragonblooded. And that's not because I like them best. They're just the ones that seem most "load bearing". Sidereals could be replaced with gods, Lunars with Raksha, and Solars with Infernals or Lunars. But DBs seem pretty irreplaceable.