I also have Marcurio as a spouse. He kept switching back to his original robes when I gave him 'Master' robes of destruction, so when he offered his Adept ones for sale I bought them thinking that would fix it. Now at home he's usually in his undies.
That's unfortunate - or not, depending on how you look at it
I think the game respawns NPCs' outfits when ALL their default apparel is taken, so if you just take his robes, he will walk around in his remaining apparel, which is just his boots. So you can force his original robes to respawn by taking his boots (make him follow you again and buy them off him). I tested it out on him just then and it seemed to have worked.
Btw if you want to replace his outfit, you have to give him something with an armor rating, as followers will only equip something with a higher armor rating than their default piece. (But he will revert to his adept robes when he's not following you.)