[Halfdeck]

I know LL owners like to get all the backlinks they can get their hands on (so would I), but you're forgetting one thing:

Duplicate content.

Getting linkbacks from supplemental pages is not going to do anyone any good.

EDIT:

Not to mention low quality backlinks from free sites aren't going to make or break your ranking on Google (though MSN probably eats them up). In a few years, who knows, Google may ignore them altogether.

One way to look at recips is advertising your LL via increasing brand awareness. Approach them as means of inflating your SE position -- and you're in violation of Google guidelines.

Preventing duplicate content is a legitimate reason for disallowing mirrors, however. A large percentage of supps under a domain may negatively impact the entire domain.