Mike,

As the others have said... it's a pain, takes room, and makes for a nice fireball on impact. First, straight propane has too much pressure, so you have to use a mix. Powermax was the standard, but it's no longer available. Also, it will still be marginal on a hot day for line pressure. Second, you still have to carry a bottle around, and you will still have to "Re-charge" the onboard tank before each flight to make sure you have enough gas for a good start. And third, the propane mix full is more expensive.

Because of those 3 reasons... why not just plug the (cheap) propane bottle on for a start? It's not like the old engines could do a re-start, so why have it onboard?

Oh... and since it went out of favor... and since it burns at a different temp (needing a different setting) if you run out while at the filed... chances are, no one will have any. But most of us have standard propane in the flight kit.