It was interesting to me that Ken selected Kelly specifically because she was clearly an outcast from her tribe. That would have been a good move normally, but since Ken was already set to have a majority of people from his original tribe still around, it only served to make them weaker overall.
I don't think that the problem is that they "fear" Ace, Jacquie, and Sugar so much as they want to make sure they have a stronger majority. I think they might fear that one of their own original tribemates might be willing to flip, so by keeping Kelly, they have that extra leverage.
Perhaps it just goes to show how paranoid the game can make you.
And I'm not sure I completely agree with you about not changing the dynamic. I agree that the stronger team, Kota, wasn't hugely affected. Charlie, Corrinne, Bob, and Marcus are still "in control" there. But Fang had a complete shift. Dan, Randy, and Susie lost Matty and are now in trouble if they ever lose a challenge. They were planning on getting rid of Ken, GC, and Crystal, who, with Matty, now control the numbers on Fang.
If Fang keeps losing, then Kelly, Sugar, and possibly Ace won't last until the merge, which will give the original Fang alliance more control than the original Kota alliance, dispite Kota having been a stronger tribe.