What's the need for higher frequency in inductive charging systems?
It all boils down to efficency.

As the frequency a transformer runs at increases, the transformer can be physically smaller. 

To Plagiarize Quote Wikipedia:


  The EMF of a transformer at a given flux density increases with
  frequency. By operating at higher frequencies, transformers can be
  physically more compact because a given core is able to transfer more
  power without reaching saturation and fewer turns are needed to
  achieve the same impedance.


Consider the size required for a 50/60 Hz transformer capable of 6.6 - 50 KW (the power rating range of the Magne-Charge systems).