Spam reports to Google serve the purpose of letting Google know what the current techniques of spam are and how sites are getting away with the spam. A spam report does not get a site demoted in the search results and only on very rare occasions does Google actually act on a spam report.
According to Google, the purpose of the spam report is:
Spam Reports are submitted directly to our engineers and are used to devise scalable solutions to fight spam.
This means that they use the reports to devise and refine the search algorithms to detect it and deal with it. They do not act manually on each and every spam report they get. Can you imagine how many staff would be needed to act on them all? That is assuming that all spam reports are legitimate and not just disgruntled webmasters complaining about competitors. Because of the sheer volume, that is why Google continues to work on refining scalable algorithmic solution to spam. That is why they need the spam reports.