I think it expands the experience immensely, myself, because the people who built the monuments walked to them. It is much easier to appreciate landscape context by walking through it!

I agree though that it has undoubtedly reduced the number of places I've been to. I do look at, for example, Gladman's epic Scottish tours with jealousy, as it's not possible to fit so many sites in unless you can move between them by car. I've still been to hundreds of sites by public transport and on foot though, often ones that no-one else bothers with, so perhaps that's the compensation. :)