IMO there are some conditions where shorts and (hairy) legs are better than full lycra which retains water and perpetuates coldness.

I wore all the kit to start but know it keeps me warm. I was a bit too warm on the first track but from Skye road onwards was fine - not too hot or cold despite my hands taking an early dunking (with both under and overgloves).
Having a survival bag made me confident enough to not carry another base layer - on long (summer weather) races I'd rather carry a survival bag than a pair of overtrousers.
I do sometimes wonder about carrying a spare baselayer when I start off in one, but I now have enough confidence in my GoreTex jacket that it will keep the layer I'm already wearing dry (save a bit of sweat) and warm.

Wearing a vest over your waterproof jacket makes it easier to show your number, but if its a breathable jacket it surely negates this property in the covered area, leading to less wicking and damper undergarments.