I recently bought a KTM 200 exc for a lightweight trail bike. It is excellent at that and I love riding it on singletrack alot more than my WR250R.
The thing that bothers me is the pollution these bikes create. I realized they smoke a little before I bought it but I didn't know that 1/4 to 1/3 of the gas goes straight out the exhaust. A government study I saw said that one hour of operation is equivalent to driving a modern car 9,000 miles as far as hydrocarbon emissions go. Some of those emissions are known carcinogens.
This can't be good for riders, other trail users or the environment. I really like the bike and honestly can't imagine riding a 4 stroke on singletrack anymore. Anyone else struggle with this or am I thinking too much?