The whole "21 days" bothers me, because what's to stop an SG from throwing Raid Matches repeatedly on the last day, so that their Items remain "in the mix of things"? Instead, why not make it a 2 week timer, and increase the timer for every successful defense of the SG? This way, we have an incentive to keep winning, so that the IoPs would remain in our bases indefinately? After all, if one SG is at the top, whats the point of fighting since you're just going to drop (you cant get any more through the trial, and you can't raid to get anymore) below others eventually anyways?
Oh, and in the CoV beta, the IoPs only gave a bonus while one was in SG mode (of course, the +1% Exp gave an extra 1 exp at very low levels, meaning up to +10% exp for us lowbies.)