ya its designed to 'share' links with out the cost of linking from your site. If you think about it thought its a pretty bad exchange.
Its like saying 'hey if you link to me from your site, end devalue it, I'll link to it from a completely different site that already linking to 100 different places and not really gonna add anything to ur site lol ur screwed'. 4 way seems more fair
b:mainsite1's link page
d:mainsite2's link page
This makes me think of twitter, how a lot of people will claim to be on 'teamfollowback' and then after a few days they unfollow everyone. you similarly have to check to make sure they're are holding up their end, and honestly it hardly seems worth the effort(unless you have automated methods)
google's webmaster tools makes it easier to track links to your site, but still, site A could stop linking to site b, and how would you even realize it, unless you thoroughly check