You are much more likely to catch your fall with your hands when you have an accident with your city bike vs a road bike.

Biking at less than 25km/h just isn't that dangerous (unless a car hits you)

being more likely to catch your fall needs to be computed against how likely the falling is, and how likely injury is. This just confuses things, much easier to look at how many injuries occur.

In the worst case the speed your head would hit the ground at would be about 15 km/h.

it's not really a theoretical problem, people actually die or experience brain damage from falling over while standing still; we know a fall of that distance is dangerous. Helmets reduce the impact and save lives and brains. The only real question is the frequency of the falls taking place from bicycles.