That spider is not automated like something from a search engine or link software where it will be hitting your site constantly.
This is a business. Exchanging links is a trade between two people (that do not know each other personally) who both hope it benefits their business in some way, nothing more. It turns out, just like everywhere else in life, some people cheat. So you have to come up with a way to protect yourself from them. I agree that manually obtaining and checking links is the way to go, but it is very time consuming, which is why a lot of webmasters use/have used link software. And it has worked as far as rankings go. Same themed links with pagerank have a big effect in Google. And I’ll bet if there wasn’t this quirk in linksmanager the people that have used them in the past would probably still be using them. I haven’t used their software, but I have searched their listings looking to email webmasters with similar sites.
“Quality site” I don’t know if this has anything to do with gambling. Imo, people looking to gamble online want to find out where they can go to do so. The affiliate sites are just passing them on to their destinations. There are sites ranked very high and I wouldn’t consider them anything great but they serve their purpose – get the searcher to his/her destination.
Basically, losing links-people not linking back will cost you pagerank. But I think maybe some of the recent losses in that area might also be Google catching up to things like spamming blog guestbooks, buying links, non-themed backlinks, etc.