I'd say this also depends. A bicycle is more stable at a higher speed. When riding in traffic (and not using a separate bike lane), I tend to not ride slowly - as it creates all sorts of problematic situations, compared to riding fast (as fast as a bike goes, 30 km/h for in city, flat land riding).

I consider road bike group rides as a high(er) risk activity, since any rider in front of me crashing increases the probability of those behing ending in a crash-pile. Solo riding I always tuck and role, but when a rider behind me crashes into me as well, it will prevent most "in-flight" manoeuvres.

This is the whole point in my opinion. Saying helmet is a must is very similar to saying it's needless. Additional problem is it's hard to objectively judge the risk, even harder to say how much is acceptable. You could say that using one is being on the safe side - and it's true. Still, exactly the same can be said about a buletproof vest - but you don't hear people saying it's silly to not wear one - the silly thing is riding (living) in an area that makes wearing one warranted... does similar go for helmets as well?