I think what he's saying is that it's been owned by a leasing company, so has probably had only one main driver from new.
It's true to say that if it's a GTI it's probably been driven hard, but even a car which has been someone's pride and joy can just go pop one day for no reason.
My GTI was a one owner, although it was a Lloyds TSB lease car. It has the fully documented history and was in good condition. It's probably not been loved as much as some, but if it breaks then i'll pay for someone to fix it?
Bottom line, if you like the car and it feels right then go for it.
Definately consider Warranty Direct, warranties from independent dealers are usually worthless.