In reply to martin_whitton:
as said, this was once in 7 years case.
cliff ~8m (I live in Finland which is kinda flat country).
belayer 10m away from cliff (yes with bottom anchor to control the direction of possible pull)
solid granite cliff - no lose rocks on cliff face or on top (we knew the crag well)
>>
Risk of something hitting the belayer (in that situation) was equal to same thing happening in indoor climbing.
One key point was that something unexpected happened (friend forgot to pack helmet). we analyzed the situation together as a team and found a way to proceed that both assessed as safe as the walk to the cliff.
Should the same happen in ice-climbing (short top rope) we would go for same setup but belaystation 15 m away from wall (well clear of possible debris)
Forgetting helmet on multipich route would (to us) mean that helmet is a) in camp >> return or b) at home - 1500+ km away which in turn means visit to local climbing store to buy one.
Climbing highlights risk assessment skills. Helmet is very high value on this. Specially while soloing. As said while soloing the direct damage is limited to climber.