That's already been discussed to death here, no pun intended As I noted last page, there are a host of safety precautions and devices people can apply to different situations. A helmet may be one part of a comprehensive plan to lessen the possibility or severity of injury while cycling.

Of course, as you say, wearing a bicycle helmet while walking might actually help prevent harm there. Doubt you'll find many takers, though Probably more realistic to make sure your shoes are tied, avoid slippery patches or uneven ground, and pay attention to where you're walking. I've seen/known multiple people to suffer serious injury from falls merely because they, by their own admission, weren't paying attention to what they were doing.