It is pretty useful for ui stuff, you can start a wait cursor in a button event, do what you need to then put the cursor back when your done, all in one thread without having to use background threads or anything.
It is pretty simple for that scenario and avoids the cross thread marshalling you need to do in win forms.
Don't forget the SOA scenario: One service calls 0..n others to fulfill a request. Each of those calls is IO bound.