Search engines (especially Google) have very diverse and sophisticated ways to determine value and relevance of a site. One overlooked by many site owners is the length of time for which the domain name is registered.
Spam sites usually are only registered for one year. Therefor a site that has a multiple year registration may be perceived as unlikely to be a spam site. Payment for several years in advance usually saves the registrant a percentage of the single year fee so it makes good sense to invest in a 5 or 10 year registration.
