Yes, stuff happens, but not too often with dynamo lights. I've had failures, but not many. Sometimes, when I'm feeling unlucky, I bring a battery flashlight just in case, but that's pretty rare. And sometimes I bring two tail lights. Two blinkies flashing at different frequency are probably a good attention getter.

One thing I've seen in NYC that I think is cute and clever is having two headlights on the sides of a very wide Wald front basket. Having two widely spaced headlights gives one a feel of perspective, since as the vehicle approaches, you can estimate the approaching speed.