Unexpected expenditures aren't usually welcome regardless of whether it's for bicycle stuff or not. I don't like having to spend when I wasn't planning to either.

Lights are important, and it's also important to have whatever you need to have a certain comfort level and confidence while you ride at night. If dynamo lights are what you want, then so be it. I always believe in redundancy and have two forward facing lights and two backward facing lights. That way if one fails, the other one still is lit. I also carry extra batteries for the lights that don't run on a rechargeable battery pack.