Especially if one suffers from acrophobia. It might be irrational but, for sufferers, they feel a sense of panic when they are at a certain height. When gripped with this fear, they are often unable to trust their sense of balance. If you are concerned for the safety of the users of these structures, what about the workers who constructed them?
Just a word of caution. Never attempt to take the ultimate selfie. It might well be the last one that you will ever take.