Why are the rear wheels of time trial bikes disc shaped rather than having spokes? Won't having disk shaped wheels cause more wind resistance with crosswinds? What are the benefits of having these sort of wheels?
Disc wheels have lower drag than spoke wheels.  See Wheelbuilder aerodynamic data and Aerodynamics of High Performance Race Bicycle Wheels.  (One link shows simulated data, the other measured).  

Most of these tests run up to yaw angles of 30 degrees.  The argument is typically that at higher yaw angles (closer to side-winds) the rider would choose a spoked wheel instead of a disc.

Disc wheels are typically preferred for courses and days that do not see a lot of cross-winds not because of drag, but because they present a large area for the wind to push on, and can lead to unpredictable performance in high-crosswinds.  This is one reason you don't often see disc front wheels in a non-velodrome environment.