Yes and no. A toilet with no vent may not flush the contents out of the bowl, but any other drain will work without a vent. (NOTE: the code is that all fixtures shall be vented). Only twice in 15 years has the vents been the cause of a drain backup. In one case it was roofers who stuffed the old roofing material down the vents and the other was just a stray piece of wood. In both cases the material made its way down into the drain pipe and had to be removed. No amount of *vent cleaning* would have done any good.