I think that this is one part of a bigger picture..........
The very presence of the police on the streets armed with water-cannon will not only (hopefully) dissuade folks from peeing in doorways, but might stop the 'flash-points' around the bars and pubs of disparate groups of drunken revellers; so in effect, they are preventing 'proper' crimes from occuring.
Admittedly, we'll have to wait until afterwards to see whether it could be claimed to be a success...........but if it means I don't have to hold my breath when passing by shop-doorways the following morning, I'm all for it :o)