Plus patches are easier with Steam anyway.
as easy as it gets really.
When I make a pilgrimage down to Fry's I look at the retail bin for specials, sometimes I get things cheaper like when I picked up Crysis 2 Collectors Edition for $35.00 as they were running a special on it.
As for Civ 5, isn't this a moot point as it's a Steamworks game so you have to install and create an account anyways ?!?