I know this has been discussed previously, but one problem is how do you know who is part of a pool? I could set up 10 pools (10 servers) with 6% each and then they'd be under the threshold individually, but I'd control them all.
The idea would be to make a Prisoner's Dilemma that would encourage defection. This would not work if you were one person who owned all 10 pools; but if the 10 pools were a loosely-maintained conglomerate, then the conglomerate would fall apart because the first group to defect from the conglomerate would get some sort of reward at the expense of the rest of the conglomerate.