Agree that, in terms of training effect, estimates are close enough and precise numbers can actually be harmful because they can lead to analysis paralysis. The problem I have is that I'm a numbers guy. Give me numbers and I'll dissect them a million different ways.
I know myself, I know what temptations I can't resist. Get me out on a long run and numbers just naturally run through my head. License plates, addresses, it doesn't matter. Give me too much data about the run and I'll analyze myself to death. This is why I think GPS would be counterproductive to me. It's also the reason I measure no routes in any way, whether bike, car, or anything else.
I'm not saying GPS is inherently bad. I think it could be a good thing in the right hands and with careful use and an understanding of its limitations. I just know my hands are not the right hands due to my number crunching nature.