First, the notion that CoS is a weak position is not really historically accurate. Under George W. Bush, that was certainly the case -- his CoS' were (by design) not powerful.
But Dick Cheney was Ford's CoS, Carter's "Karl Rove," Hamilton Jordan was his CoS, and James Baker was Reagan's CoS (just to name a few). These were powerful leaders who had major influence.
The position can be very, very powerful -- and my suspicion is that Rahm will be a key player in an Obama Administration.