Well, the sponge itself would be rinsed of fecal matter in running water, and the salt water/vinegar soak would have taken care of the rest. Smell (at least, the smell of the wiped Romans) wouldn’t have been a problem - transmission of parasite eggs from fecal matter, on the other hand? Probably a non-negligible issue.
https://www.livescience.com/53303-ancient-rome-infested-with-parasites.html
It was pretty bad.
Pre-germ-theory food handling and water source management always took a toll, so I’m guessing poo sponges were a relatively small part of the problem. Especially if the vinegar soak actually worked at all.
He also reviewed studies analyzing Rome’s ectoparasites — that is, parasites found on the outside of the body, such as fleas, lice and bedbugs — in textiles and combs.
Surprisingly, ectoparasites were just as common in the Roman Empire, where people regularly bathed, as they were in Viking and medieval populations — groups of people who didn’t bathe frequently, Mitchell found.
Now that is surprising. How hard is it to just delouse your shit before you put it back on?
Surprisingly hard. Delousing was a major endeavor all the way up into the 20th century. The little bastards are hard to completely eradicate.