I can't see the argument for not using it? Its been proven that accidents have serious consequences when they aren't worn and people can unexpectedly leave the helm position without falling out of the boat and still leave it out of control putting life at risk. I work full time as a skipper as well as instructor, inspector and examiner and wear it 100% of the time if the boat has one even for the shortest and slowest trip. If it interferes with what I'm trying to do I replan what I intend to do rather than carry on without it even for berthing etc. Manufacturers fit them as a safety device to prevent serious injury not as something to blow in the wind, secure the keys to the boat or cause an inconvenience when used correctly.
Here's one I found recently on an inspection fitted to a 10m cabin rib in the Italy. When questioned why it was like this 'it's so that the keys never get lost when we take them out of the boat'. It was promptly modified with a pair of scissors by myself.
RYA Instructor, Trainer, Examiner and Skipper