First, working with the police as an informant doesn't come with many guarantees, but there are some rough rules of thumb: You can rely on the prosecutor dismissing a charge for every two arrests of equal or greater value. Get busted for possessing and intending to distribute cocaine? You'd better be able to help the police bust two other coke dealers. Two-for-one is a great deal whether you're talking about felonies or Little Caesars pizzas, which is why cops are so willing to work with young ne'er-do-wells like our source.
"We'll even take ruffians and no-goodniks."
But already you can start to see how messed up this is. For one, the police now have motivation for you to stay in that criminal life -- the moment you go straight, they lose their leverage and your valuable connection to the underworld. If you watched The Wire, this might not be a big surprise. It sounds exactly like how the Baltimore PD used Bubbles:
AKA the most beloved heroin addict without his own name on a star in Hollywood.
Think about it: If the only interaction you had with bad guys were ones that resulted in them all getting arrested a few weeks later, how long would it take them to figure it out? No, it only works if they know you're part of the scene, which means you're going to be routinely doing the things that your tips are getting other people busted for. You can get caught doing heroin and truthfully say, "I'm doing this for justice!"
And if you're wondering ...