I've seen a couple of articles recently suggesting that (a) sponges are too expensive to use as toilet paper and (b) that sponges would not remain reliably attached to the stick.
As merely an interested amateur I can't speak to the relative cost of Roman sponges compared with the utility of the end product and I have no intention of attempting experimental archaeology to test the thesis that they would not be robust enough.
I remain intrigued none the less.