Imagine the worst, most foul thing you have ever smelled. An overpowering mix of rotting meat, old socks that haven't been washed for weeks - topped off with the pungent waft of an open sewer.
Imagine being covered in the stuff as it is liberally sprayed from a water cannon.
Then imagine not being able to get rid of the stench for at least three days, no matter how often you try to scrub yourself clean.
Skunk is made from yeast, baking soda, and some secret ingredients that are completely organic. David Ben Harosh, an Israeli police superintendent, is proud of the Skunk project.
"It's totally harmless, you can even drink it,"
Link -via Metafilter
Simple. Because it won't kill the civilian (and that is inevitably who it is used against, citizens), police tend to use it more and more often, for lesser and lesser reasons. Eventually, they are used as punishment, again, for lesser and lesser reasons. See the abuse of tasers in the US.
This kind of thing should only be used on the battlefield. Civilians, no matter how belligerent, shouldn't be subject to such attacks. Either shoot them or arrest them. Don't punish people without a trial.
Oh you know the officier manning the water cannon is totally gonna be aiming for people's mouths.