I like reports for two main reasons:
1) Reports save time by directing moderators to spammy messages.
2) Reports also helps us mods gauge how the other members view spam and allow us to apply better judgment on how to handle them.
The obvious spam will almost always be found and cleared within 24 hours, so don't worry too much if those aren't reported. Personally, I find the following reports most useful:
a) Reports of much older spams (more than a year) that people stumble across. A spam from five years ago can still do damage because it is still being used by search engines to boost page rankings and search results, and they can point to viruses and other malware. All spam must die.
b) Obscure or "odd" stuff that may be the precursor to spam, or just someone playing games. These can be overlooked by moderators on spam-patrol because they are often normal-looking messages at first glance.
c) Any spam older than a week - this was either missed, or the account holder had a delayed-fuse spam bomb in his/her/its signature that was triggered at a date weeks or months from when the message was first posted.