Is my Salesman telling the Truth?

I stopped by my local hifi salon yesterday and my sales guy was blabbing how much cardas sucks and taralabs are so much better. Since the topic of cable preference is a very personal/subjective issue, I decided not to touch it with a 3 meter pole. He then goes on to say that network cables such as Transparent and MIT doesn't do a thing for single wire post speakers (non-biwire). I understand that most sales guys are knowledgeable and would never lie to make a sale (yeah right), but can some one comfirm or explain why this is? Thanks in advance.