Why do u need a carbon stem Jock?

Our local club president has a spanking new Giant TCR Advanced (the kind with the integrated seat post that you trim to size) that has been recalled to replace the standard carbon stem with an Aluminium one after some poor bugger in Oz had the original stem break in a fairly major (and painful) fashion. I can only assume the fork has a full carbon steerer given the rest of the bling on the bike. As an aside he has made a couple of minor upgrades such as replacing the DA brakes with Zero Gravity jobs ($$$$$$$$$!!!)