Dave, one of the advantages of being older with a mushy brain (had a 5 year old ski racer tell me last week that I was forgiven for not remembering his name from six months prior due to my being old and having a mushy brain)
is that I have seen a fire fueled at a crash site due to propane
tank on a trailer being left on. Not to mention wire shorts in trailers can happen at anytime - parked or driving and I would rather not look back and see I am pulling a ball of fire. Besides even if it is a slight chance of an accident it still doesnt fix my old mushy brain and help me to remember to turn the tank off when I pull into a gas station.
Its just my personal comfort choose and you did ask why some of us don't tow with the propane on. Not everyone will agree with my choose or reason for it.
Edit to add: Although the odds may be low as to an event actually happening - it can happen. Having had an event take place which took a family members life with odds of it happening of 11 million to 1, so I am probable more cautionary than some when it comes to playing odds. ;-)