Why are they? If I've got 1 or 2 commercial inbounds there's nothing wrong with extending the light aircraft downwind, tell him who he's following and remind him of the number of miles recommended spacing. The relative speeds are such that the C152 (forf example) won't go that far downwind.
I agree with BackPacker.
If I have 3 airliners on the ILS, and 3 airliners taxing out, I would definitely not extend downwind leg for a light aircraft to come in behind the last one on the ILS. It is so much easier to keep circuit traffic close to the airport. Right hand orbits (when left hand circuit) abeam threshold is perfect in my opinion. The circuit aircraft would probably be overtaken by the next batch of arrivals if extending it's downwind to 10 NM. Extending downwind works better if all aircraft are doing equal speeds.
We have had a few TCAS problems with light aircraft on extended downwind. Airliner pilots don't seem to like "opposite" traffic at same level on 3 miles final...