The benefits of cleaning regularly far outweigh the miniscule impact of a few hundred gallons of water and dirt that might go straight down the storm sewer.
It's time we use some common sense and quit being led around by people who use the government to sell their products (or orgs).
Take the fine lobbying work of this urinal manufacturer for example:
Palm Beach County, Florida, schools installed waterless urinals a few years ago. Officials said the urinals were better for the environment and should save about $100 per urinal each year in water costs. But the pipes behind urinals in at least two high schools corroded, causing urine to leak into the walls and onto the floors. Repairing the damage and replacing all of the urinals with urinals that flush will cost about $500,000.
Just because it "sounds" environmentally friendly and just because it "feels" good to do doesn't mean it makes any sense. And that includes spending more energy, air pollution, time, taxpayers money, and all sorts of other things to combat plaza and sidewalk runoff that isn't a problem in the first place.