Climbing helmets seem only to be worn by Bigwall and Trad climbers, the helmets I assume serves two purposes:
1) protect the lead climbers noggin in case of a high factor fall.
2)protect the belayer from all sorts of objects, ranging from gear to loose rocks, reaching terminal velocity just before rendezvousing with said noggin
Although the risk in sport climbing 'seems' neglect-able, surely it is wise for the person who controls the life of the lead climber to wear suitable headgear. Again I assume that the problem is a similar one as to the one that the USA army faced in the first years of its invasion of Iraq i.e. huge amounts of eye injuries caused by shrapnel due to the spec military goggles being uncomfortable and more importantly uncool... they solved the problem by getting a company to produce smaller/cooler looking safety goggles.
I propose the same thing, there has been a recent development in industrial 'bumpcaps' it offers first base protection and is allowed for certain applications instead of a full helmet i.e. it would for instance be allowed inside ships engine compartments instead of a full helmet. It comes in the form of a plastic inner on a baseball cap (see pic) I have removed my inner and can used it with almost any type of hat (peak cap naturally the best) it does not smother the air flow on your head and it almost like wearing no protection at all.
I think that this cap will work very nicely as a purely sport climbing belayer's alternative head protection, see pics on how nicely it fits in my sharks cap.
P.s If you feel that my earlier assumptions about helmet usage were wrong feel free to correct me.

