The age-old motto, "always be prepared" has particular application to cycling. You don't want to have an accident and you don't want to get stranded, but both unfortunate events can happen. If they do, you'll want to be able to call for help, so bring your cell phone. Many smart phones have GPS technology or the capability to access the Internet for a map. If there's been an accident, you can summon emergency help.
It's also a good idea to pack identification and a debit card or small amount of cash. Money and an ID can be absolutely vital when you're in a tight spot.
When you consider the light weight of a debit card, ID and cell phone, there's really no reason why you shouldn't bring them [source: GORP].