It's just not as profitable as you might think. Think about it. The only time when it would save you a significant amount of money is when you have a very good hand but your colluding friend has a better (e.g. AA vs AK on a Kxx flop) and the chances of this occuring aren't that high. Any other edge (say being on a flush draw and your friend telling you he had 1 of your suit) isn't really a significant edge. I doubt colluding is really that profitable, especially since the players that would do it are generally going to be low limit players. If you played any higher limits, well, are you willing to trust your 'friend' with $3000 or something? There's no way to get that money back if he tries to screw you.