There is the odd detail here [webmasterworld.com] and here [webmasterworld.com], as well as in another forum site.
Basically, dec owns AV or vice-versa, and Mercator is supposedly used by a research branch investigating the linked nature of the web. Personally, I think that AV is trying to get some sort of link detail so it can rank pages like Google. Mercator visits my pages a lot.
I agree that they are just checking the linkage on various sites as our server logs show that they do not actually "retrieve and/or store" any of our data. I've seen a few other companies doing the same thing and they state thay they are mainly checking the integrity of links around the web while also checking response time. Maybe Mercator is doing something similar