What are the drawbacks of not attaching deck to house?
Doing this has the advantage of not having to attach a ledger board to the house, which must be flashed carefully to keep water from getting in. 

The big disadvantage is that you don't get stability of hooking to a big immovable house, so if the deck is a second-floor one, you will probably want more diagonal bracing on the posts to keep things stable. If it's a low deck, this probably doesn't matter.

So, no real reason not to do it detached. There are a number of builders who do it this way.