I always carry full kit irrespective of requirements (I'll leave out the trousers on short/low races, and even then only if not specified by the RO), but since I'll never see the sharp end of a race I don't really give a monkey's what other people do - although I do hope that maybe one of these people goes over on their ankle at the furthest/highest point from civilisation and has to limp 5 miles back while it clouds over, starts to rain and the wind gets up and they're just in a vest...
[and they can bite the fattest part of my @ss if they think I'm lending them mine]