I'll give it a stab and try to give a reason.

Backfitting is a problem when the rules you add are not aded for a logical reason. The more rules you have the more likely that you are backfitting some of those rules.

The other reason is that usully the more rules you add to the system, the more you restrict the result set. Less results mean less confidence.

I can find many systems that only have 2 rules that make 5% plus but I have to say that variance is huge in these systems. You go from -20% to +20% swings and the average punter couldn't handle them(me included). So I think adding a couple of extra rules to minimise the variance is acceptable.