Generally if directs are available we give them to you. The sooner we can get you out of the main flow of traffic the better for the both of us. The usual reason we cannot give the direct is becuase of military activity. Sometimes we can make a call to the mil's and they might let us fly through, it just depends on what they have flying at the time. If we can see them on the radar we just don't bother.
On a personal note I don't mind you asking for directs but when the freq is obviously busy, it is a call we can do without. Chances are you'll block another call and we probably can't offer you anything anyhow.