Most police informants are criminals who will wind up doing time unless they give the police useful information.
People can wear all the "Stop Snitching" t-shirts they want, but when the cops say: "Look, pal, you're going upstate for 5 years unless you tell us who sold you the coke" they're saying who sold them the coke and they're leaving town.
Unfortunately there are too many cases where the informants keep giving information when they no longer have any valid information to give. There have been too many cases of innocent people suffering "dynamic entries" and arrests because some crook had to keep "naming names" or he'd end up behind bars.