So what about the other permanent thralls? Flame svck. Frost are too huge to be in dungeons with you (and svck too for their damage output). Storms are much better and probably the ONLY more useful conjuration than dremoras for dragons but storm thralls do not regenerate their health either. But even so, dremora lords are way better than storm atronachs for any battle situation besides dragons or ranged opponents that cannot be reached. Those situations are always exceptional tho...so dremora lord for practicality still pwns the storm atronach.
Now what about these ridiculous conjurations Dawnguard and DLC added? Truly I mean ridiculous especially with DLC since NONE of the new ones like Seekers or Ash Spawns are affected by the conjuration tree for twin souls or duration. And then on top of that...they still stink compared to dremora lords! Seekers don't do near the damage and have roughly the same health. Same with ash spawns.
Now what about Dawnguard? Mistman? Give me a break. Boneman? As horrible as the skeletons you fight that you can kill with a few hits with your bare fists. Wrathman? Now HERE we got something finally! The Wrathman is actually about just as strong as the dremora when you compare its weaker damage (but with dragonbone weapons it's still decent) but much higher health. Indeed Wrathman would be more useful than dremoras for players who want summons that can tank really well.
But even so for the high level players (so long term wise) who fight on high difficulties and summons cost them hardly any magic, dremoras are still way better than Wrathman practicality wise again for their sheer damage output. And it's not like a Wrathman is any different with a fighting style compared to a dremora. Only difference is less damage but more health.
So there you go. I suppose the ONLY other conjuration that has a sufficiently different style than a dremora but equally as good in its own way for battle purposes is the Ash Guardian. But you can't conjure it friendly without mining all day long to get tons of freaking heart stones!! How stupid is that? If it costs something to summon an ash guardian then it should certainly cost you something else to summon dremoras. Now truthfully the ash guardian is still useful to use an store heart stones with whenever you want THREE conjurations (you can conjure an ash guardian beyond the twin souls perks so you can three summons out using it) but that's again only for exceptional circumstances.
So..there is only a couple of other summons that have better use than dremoras for EXCEPTIONAL circumstances. Over all though, duel casting dremora lords pwns absolutely all other conjurations.
My big takeaway is I still love dremora lords and will continue gleefully watching them decapitate guards screaming "No match at all!" but Bethseda did a horrible job with conjuration VARIETY once you get over level 50 because duel casting two dremora lords is king over pretty much anything.