But why carry around the extra weight of a heavy duty inner-tube?
Since writing the above I have had time to experiment with low pressure inner-tube and tyre combinations.
Here are my conclusions so far:
On smooth surfaces it only makes sense in terms of increasing tyre contact patch and so the grip. On rough surfaces, like cobble stones, the low pressure tyres can absorb all the vibration and so reduce the rolling resistance. Tyre design is important in that an inflexible tyre will have a very high rolling resistance when run at low pressure. The original Cleland tyres are thick but flexible, and are remarkably quick off-road.
1/Using ordinary "tubed" tyres on tubeless rims stops rim creep and the resulting valve rip out.
2/ Some tyres have a reasonable rolling resistance when run at low pressures. However some that have good high pressure rolling resistances are very bad when run low.
3/It is by matching the width of the tyre with the width of the tube that allows the use of very low pressures. Wide lightweight tubes appear to work just as well as heavy duty tubes. (Too wide and the tube will chaff and eventually puncture. Too narrow and too much pressure is wasted on expanding the tube to contact the tyre).
So a heavy-duty, thick walled tube, is only useful in that it can prevent pinch punctures and reduce the risk of ordinery punctures. (It should be possible to redesign rims so they are less likely to pinch puncture the tubes. Also, some modern cars are designed to run flat for short distances).