Gopher to the Sleuth Gods
I ask a townie "what have you heard about this case?" If I do that to expand my suspect list it is helpful. But to ask that just because I can, is the answer relevent? They may say Mr. X knows, I ask Mr. X about the person I know is guilty and he says no, I don't think its him. Why bother with it?
Yeah, beyond building the suspect list, it's not all that useful. Couple of things though, if you know that a particular person has no suspicions, you know you aren't going to question them about that, so you don't have to worry about wasting valuable questions on them...so you can ask them for motive and alibi more freely because from that point on they're essentially useless.
Also, the "so and so thinks they know" can be helpful in choosing who to try to unclam with surveillance if you have that skill and find yourself out of options. The key word there though is "THINKS"...doesn't mean they're right :)
That said, the only townie I ask is my snitch contact; the rest I'd much rather save to check alibis if they come up.