I can't believe that THAT many people are running carbon hoops, I certainly don't see many at all in the 2 cross leagues I race in (London League and Easters (read: Essex) League) here in the UK.
Perhaps just a regional difference. In North America I would say 90% of the Elite Men/Women are on carbon wheels, and even half of the beginners. Most people glue up their road race wheels for cross in the fall, and lots of the 35+ crowd have the money for whatever they want to ride.
Carbon or not, the road inserts make changing pads quick (less than 60 seconds per wheel) which is the real advantage. No need to readjust everything since the pad holder maintains its position.