These just move the exhaust from the tailpipe to the smoke stack of the coal fired plant.
Not that getting the emissions out of the population centers isn't benefit enough, but it's also a lot easier to fix a single scrubber to a coal plant to reduce NOX than a thousand to cars.
Plus, you are helping to create support for a new distribution system, one that can be switched over to electricity generated from cleaner options, like solar, wind, biomass or burning human flesh.