Your arguments seem to be based around that idea that players won't be clever in their use of Dominate. To the contrary, I find that most players will be extremely creative and innovative with the use of their character's abilities.
No you don't want a permanently dominated army around, but for extreme short term dominating a lot of people at need can be darn useful. I'm not even talking about for combat. Need to get something hidden somewhere in a castle? Dominate somebody who works there and have them tell you all about the layout. Want to ambush somebody? Dominate a few of his flunkies and servants and have them slip poison into his wine then lead him into a trap. Need to establish a cover identity for a few days? Dominate somebody and have them vouch for you as their long-lost cousin.
I defy you to lay out for me an adventure involving humanoids where the ability to lay out Dominates at will won't help. No it's not a combat ability primarily. Combat is only a small component of its many uses.
As for Vampire Spawn, you don't want them for the long term. However with a little time to prepare they can still be pretty potent cannon fodder. You keep talking about "CR 10 challenges", but remember that CR is calculated for an open battlefield fighting on relatively even terms. Lost of vampire abilities give PCs the ability to do "set up work" that will tip the odds in their favor when the fight finally occurs.
Again, what a lack of creativity! You don't dominate the shopkeeper. You dominate the daughter of the shopkeeper, have her swipe the stuff, and then slit her throat in a thoroughly non-vampiric fashion afterwards. No muss and no way to trace it to your undead activities.
But that's just off the top of my head. Give me a few days to think about it, as the player of a vampire PC will, and I'd come up with something better and designed to account for available divinatory magic.
Man, Dominate Person at will + Gaseous Form at will could lead to so many great scams and tricks.