When was the last time Microsoft got listed in surbl ?
Smaller lists might end up being sent from a false positive domain and the idea is that surbl test pattern (queries/minutes, burst/continuous, historical comparisons, geolocation and perhaps other metrics) should allow to differentiate between such a list and a spam run.
Spammers could add some fake URIs like yahoo.com, gmail.com, microsoft.com to their spam runs so that their mails get a hammy score(if surbl gives a negative score using some whitelisted URIs). Also, spammers could use a badly configured good intentioned mailing list like sourceforge.net or through services like yahoo.com, gmail.com etc could reduce the accuracy. Having a grey +ve score for URIs queried from MTAs with patterns matching a spam run is a nice idea though.
-- Skar.