I think we need to clarify what someone would call an aero bike and the weight that goes with that and what a light bike is. I do not see why you can not have both.
A 900g aero frame will be faster than a 900g non aero frame.
A 1100g aero wheelset will be faster than a 1100g non aero wheelset.
So why would you not go aero and light?
Because a 900g "aero frameset" will be compromised in order to get the weight to that level.
Take the Foil. It weighs 940g. The fork 330g. That's around 240g more than the 2012 weenie framesets.
Now if you ride at 40 to 50kph for two hours you might still benefit. Otherwise it's pretty pointless really. Or so it seems. The trouble is no one has come up with a convincing demonstration of objective benefits for road riders when it comes to aero.
I for instance tend to average around 30kph on rolling terrain when not climbing mountains. At such slow speed ( although it's far more typical of all non racers ime) the so called benefits of aero are reduced to around 20% of those quoted as I understand it.
So taking a Cervelo S2 over an R5 would gain me something like twelve seconds in two hours of riding. I could stand up to fart and lose more than that.