We've chosen air cooled over water cooled for the following reasons...
If a head gasket fails on a water cooled bike you are stuck usually with water in the oil. This is not good. The latest crop of Enduro bikes do not have the pedigree of either the XT600E or the DR650. Their frames are more flimsy and whilst Walter Colebatch proclaims the X-Challenge (which doesn't appear to be made anymore) is the perfect bike I am bemused by the number of failures/breakages he has had. My XT is doing approx 70mpg. My Ural Cross sidecar outfit, that I rode from Plymouth to cross Mongolia and return last year - unaided, was aircooled and did approx 39mpg even when loaded. The only problem I had with it was a cracked (from new) rear drive unit and the failure of the Slovakian built sidecar sub-frame. Yes the bike could be more energy efficient but at the cost of, in my opinion, simplicity.
I don't want ABS, fuel injection, turbo-charging, electric fuel pumps, or water-cooling.
If my carb blocks - I can repair it
If my headgasket fails - I'll get a bit of oil on my leg (by the way, I can repair it)
If my radiator punctures - Oh! Wait, I haven't got one.

It has been stated that the water cooling only adds 1.5kgs. Hah! Add the weight of this lot..
Radiator. the water in it, hoses and clips, thermostat, radiator fan, thermocouple to trigger the fan, electric fuel pump (which on the KTM adventure 990 takes 2 litres of space in the tank), A temperature sensor (to frighten the crap out of you when your engine overheats), perhaps a temperature guage... That lot weighs a damn sight more than 1.5kgs/3.3lbs.
Nope, I'm not a Luddite, I have all the latest tech at home and I will have GPS, a Netbook, 2 digital cameras, a mobile phone, and an air-cooled XT600 because I don't want to have to fix my water-pump in Siberia/Mongolia or wherever.
By the by, for all you Rotax engined riders out there, remember that the impeller that breaks inside the waterpump doesn't come with the repair kit! My friend Clyde got stuck in Aguinskoi (1 version of possible English spelling) for 21 days whilst BMW in America sent the impeller/s to him by Courier. He already had the repair kit. BMW in Moscow had the part but wouldn't send it as they said it would not get to him. That's why Clyde ordered it from the US of A.
I respect your decision to justify whatever bike you want to take and if it breaks down and I'm there I would happily help you repair it (I'm an Engineer). But don't lie to me about the advantages of taking a modern water-cooled, fuel injected bike.
I thank you
Andy