Most trackers who pursue traders just do so because it's the 'thing' to do. A standard has been set, and they want to fit in, because doing otherwise could get them 'excluded'. Some of the trackers that are the most active in trader hunting aren't even all that popular or difficult to get into through normal means, hell they even hold open signups. I'm not saying all are like that, personally I'm against trading, but a lot of the trader hunting is just to fit in or feel important. Personally I don't like it because I feel each account should be a single individual, and I've recently been having doubts about global banning just for trading invites. Sure, places don't want to be used as currency, and they want to ensure their member quality, but the chance of a terrible member getting in via normal invitation means is just as likely as one trading in. Often people just don't realize it, it's not really uncommon for friends from IRC chat rooms and stuff that aren't really hardcore into the torrent scene to just go 'Hey, I've got this invite, and you've got that one, how about we swap?' I mean people who constantly do it on trading forums to 'move up the ladder' obviously don't have a whole lot of respect for the rules, but people who do things like I mentioned above just don't know any better. The only reason I'm bringing that little rant up is because it's not uncommon these days to see someone who says something along the lines of 'Yeah I gave my friend x and he gave me y' in an interview for a site or something to get shit on.
I'm a little mixed on public giveaways as well. I'm completely against ones that require no sort of proofs because you have no real idea whether that person is capable, but I also feel it's somewhat exclusionary for those who are not on any private trackers or may have slow connections and are great members in the long run, and it's also not terribly hard to pass on fake proofs especially if the person has no account on the sites you're using proofs from.
Overall, none of them *really* put most sites at risk, the only risk is mainly involved in the smaller, closer communities, and essentially 'unkowns' getting in that they may not want in their closed community.
I will say I have no reservations or tolerance for cheating and invite selling though.
tl;dr: Just some random person expressing their views that you probably don't care about.