Since 2009 Red Bull has used a fairing in different formats to cover the half shaft, the track rod (for adjusting toe angle) and half of the lower rear wishbone. The reason for this is that the rotation of the half shaft produces downforce because of the Magnus Effect, which is good. The problem is that when you introduce the exhaust plume into the equation, this effect becomes more unstable as exhaust flow is given out by the engine only on every exhaust stroke.
In addition, covering them will provide a clearer path for the airflow at the rear of the car allowing the diffuser to produce more downforce.
This year Ferrari, McLaren, Force India, Sauber, Lotus and Mercedes (up to the day of posting this) have followed Red Bull's path of using the half shaft shroud as it is more aerodynamic than having them separate.