Knees and hands are usually the first bits of your body to hit the deck. I don’t wear knee protectors, but I have thought about getting some. I’m put off by the fact they would be uncomfortable.
The place where they seem a very good idea indeed would be the Alps (or anywhere else with rocks/loose shale etc). I’ve seen enough people with claret running down their legs after a fall in the Alps to realise that these are nigh on essential.
I had a very minor accident on a slight gravelly incline in the Dordogne last year (accidentally knocked the shifter for my front mech whilst pedalling under load). My whole weight landed on my knee, falling from about 3ft – ouch! I was digging bits of gravel out and there was blood everywhere. So you decide.