Aside on being an exchange, there still lots of fellas who do make use of as an exchange wallet which isnt really a suggested thing to be done.
Nothing to do with Coinbase itself as much as I dislike them, although the fact that they do advertise themselves as a place to keep and store bitcoin (plus they used to do the $10 free bitcoin bounty) meant a whole lot of Americans used Coinbase as their actual wallet. Don't know if this remained the case today though.