DJ.
I spend 50% of my time SUP surfing and 50% either ocean paddling or flatwater paddling.
I always wear a legrope (hate leash) when surfing or ocean paddling. I see this as a safety issue for myself when ocean paddling and others when SUP surfing.
I never wear a legrope when flatwater paddling.
I agree that if you are paddling adjacent to trees or submerged obsticals your legrope could become tangled and cause a safety issue for yourself.
I often paddle in or out of Noosa river, there is a section that has a very fast current with submerged tress that have been washed in. A trail;ing legrope could easily be caught on these obstacles. with the strength of the current you could be pulled off your board and have some difficulty.
I think a plan needs to be in your head at all times as to what you would do in a situation. Given you can't plan for evrything, but some prior thought is always handy tool.