However, having played a fair but of Powerplay now, I'm coming to the conclusion that it would be better if it was done in open play only (or possibly groups). It's not a fair playing field when solo actions can affect the outcome of the cycle (I understand that all powers have the same option of using solo).
It is fair, as far as advancing one's own power goes at least.
Two persons in Solo, each playing for a different power, will keep each one in check by each contributing to his or her power.
So, what happens when the two are in Open, able to see and interfere with each other? They will be each contributing to his or her power by preventing someone that backs a different power from contributing.
Saying that people in Open contribute less because they are hindered by players opposing them is like saying that, in a team sport, the goalkeeper doesn't contribute. They both contribute, just in a different way, one that is harder to track but just as important.
Plus, if the player in Open is alone because all the opposition is in Solo, he has his instance just for himself and can also directly contribute just as if he was in Solo himself. Just like goalkeepers have been known to score for their teams when, for whichever reason, the whole field cleared before them.