Based on the fact that it will resolve slower...being an international service.
Your ISP's DNS will more than likely be only one or two hops away, Google DNS on the other hand is 15 hops away (on my current connection).
All of that being said I've tested it with "seat of the pants" feel before and the difference generally is imperceptible.
So what is your argument? That I visit more international sites than local? I don't, neither does the company line I manage, it's more a case of 70% local and 30% international.
Therefore there is a speed increase by using a local DNS server. Or else my ISP would stick me on a international one and forego their own.