It's not enough to have cops testify that they personally saw him throw it out the window?
Besides, what does it cost to take some prints? Not much. A few minutes, and a few dollars' worth of materials.
In any case, I was addressing the topic in general, not any one specific case. If you've got the perp's prints on the object in question, you've got some rock-solid evidence. It takes away nagging doubts from the jury's mind, and that's a GOOD thing.
And we all know that no cop would ever dream about lying in a drug case.