Lurk - There are ways of keeping informants' names secret, even after the indictments. It involves the language in the affidavits, such that even unnamed sources can be qualified as reliable. Their names may only be revealed under extraordinary circumstances. However, if I fail to find the evidence in a search warrant or arrest, then my only option for prosecution might be to expose the informant. With a particularly vulnerable informant, or one who adamantly refuses to testify, the case just goes away, and we have to wait for the next time.

So I could watch, (or even videotape) my informants going into the dealer's house to buy heroin two or three times. I can get a search warrant based on the controlled buys and other corroborating evidence. But if I search his house, and his stash is hidden somewhere else, or he sold the last of it the night before, then I got nothin', and the dealer is "innocent." I have all the proof I need for my own certain knowledge, but none of it is useable in court without the informant's testimony.