The nice thing about visiting your favourite out of the way site is that it's rough round the edges, not full of people, devoid of macdonalds wrappers. You can appreciate it for the setting and the atmosphere and the 'genius loci' and whatever. But because it was constructed by people in the past, for a certain reason, you're kind of missing something unless you do know something about the site's supposed purpose? even if it's just its origins according to local folklore? It's not that I necessarily want an english heritage board outside every circle, but it's that extra information that makes you feel the continuity of people being at that site.
If these 'big' sites have got decent visitor centres (not just shops selling tat) then it's spreading the word so to speak, and people will understand what they're looking at, and they might want to look after other sites around the country too. But I suppose it's difficult deciding what your visitor centre should be like.

I know, I'm probably talking weasel droppings, I suppose it's obvious I don't even know what I'm trying to say. I think I'm trying to say that visitor centres have their place, so long as they're done with minimum tat, and are kept to sites that erm already have them. Or something.