I was playing at Yahoo when they implemented a ladder system. It started off being fun, then quite a lot of cheating started up, players were throwing games to each other to maintain their positions...they reset the ladder every 3 months, but this didnt stop them, they usually held the top 10-20 positions, and took it in turns to be #1, yes they accepted challenges, but very selectively, it became such a joke, and also became very nasty, I stopped playing there, because, really what was the point?
I have seen a succesful ladder system run at cases leagues, that can have some pretty strict rules implemented, but yet again I have seen the top positions manipulated in one league, this went on for ages until one of the players fessed up about it. Its fine to say that players have to accept challenges to keep ranking, but they can be selective in which challenges they choose, you can't very well make players accept EVERY challenge.
The thing which helped the ladder leagues at Cases work is simply you can throw out the cheats, but it takes a lot of dedicated time and energy to maintain the ladder properly.

The only experience I had was a cases league and I agree the rules were strict which is why I would think if people had to resort to cheating to maintain their position, the better players would eventually pass them up on requires accepts. I guess cheating goes on in almost everything, but they ultimately are cheating theirselves since they have to live with theirselves and their lying.