This is extremely annoying for me. It forces you to give up an item slot and the Thalmor Robes are one of the coolest armor sets in the game. It's the little things in Skyrim that make it so great. So please, separate the hood from the Thalmor Robes and if you're feeling really generous make them purchasable in solitude or some place like that. Hopefully an unenchanted version as well.
I really like this game but there are a few little quirks that really throw me for a loop.
P.S. I know there are hoodless robes, I like the hood, I just want them to be separate items.