I used to always have one if I was carrying a bag of any size, as a just in case thing (NYC resident). You have a lot of opportunities to gain elevation and look down straightaways, as well as counter-observation on someone else in those spots. From a long city block away, you'll be able to tell a lot more about that clump of people (what they're wearing, what their signs say, etc.) than unaided. I figured it was peace of mind in case of an emergency (9/11, the blackout in '04, Sandy, I and a lot of folks I know ended up doing more walking than normal). It was also handy when some cute young thing wanted to see fireworks a little better.
It has since moved into the BOB. But for twenty bucks...