If it drives more people to the online version of Gmail then that's still a win for Gmail in some sense (if not monetarily). I currently use Gmail via IMAP so never see any ads but Rapportive could tempt me back online.
We believe the context we provide is a much more valuable use of the limited sidebar space than the ads it replaces; and we think that adds value to the platform, both for Google and for its users.
We're not interested in cheating Google out of their ad revenue, although we suspect the Gmail ads aren't making that much revenue anyway.