I don't juggle.
But i think its because wind blows above them, like a feather.
Gravitational pulls are all equal no matter what the mass how ever if air is being blown from under of sides the gravitational pull is disturbed
Like you said, you don't juggle, so the last part of your statement really doesn't make sense in this sort of topic. Which I think you realized, as you did put the big smiley face. None the less, it was an interesting theory.
Personally, I wouldn't waste time trying to juggle sponge balls, as lay audiences probably wouldn't care that they are more difficult. For magicians, it may be impressive, but I can't really see it impressing a lay audience any more than juggling regular balls.