Fist of all, when it rains it pours and the weather forecasts are not always reliable (as we all know, right?). You may need it.
Carrying an umbrella may also (apart from having a close combat weapon at hand, which you may or may not need) prevent Murphy’s law from striking: Yesterday, or example, I was smart enough to bring one to a date. As I left the house, the sky was dark and it looked like it would start to rain. But as soon as my date and I had reached our limit of beer in a local dive bar in Clinton Hill (http://www.yelp.de/biz/the-fulton-grand-brooklyn – cool place with a huge beer selection) and decided to walk through the neighborhood (my date was nice enough to fill me in on the history and possible things to do) the sky has cleared. And it has been sunny ever since. But my umbrella is still in my backback.
Another side note: You may want to check out possible umbrella vendors early on: The rain on Thursday was so strong that it left the streets cluttered with broken umbrellas – victims of a lost battle against nature.