I do, when they're particularly annoying.

If I think the spammer is legitimate enough that a complaint will make it harder for them to spam, I do.

If I just want to pretend to be a volunteer junior deputy for the Sheriff of the Internet for two minutes, I'll do it.

Spammers would not have their accounts suspended as often or as quickly if no one ever reported them to abuse@some.service.provider.com . There's always the possibility that my iota of caring generated the lead that sparked the investigation that allowed the actual network security guard to take down the spammer kingpin or a portion of his botnet.

Mostly, it's when I just want to kick the spammer squar in the danglies, for annoying me when I'm bored.