I started as an affiliate just a month ago, so my experience is close to nothing. Personally I would NEVER EVER sign up with a program that carries over negative balances. Even my tiny experience confirms my point of view.
One my referred customer won 2 jackpots within a two-week period, totaling over $6,000 while his purchases were $2,500. If it was not for the non-carrying forward affiliate program, I would have been in a $3,500 hole! For just a novice affiliate with only 2 playing customers how many more months would I need to spend commission-free if it was a carry over program?!
I do not see any advantage in the carry over system from an affiliate point of view.