Well, if you want redundancy, then a dynamo + battery light would have the redundancy of 2 lights, plus the backup of that the dynamo light (if not damaged) has no battery stuff. And the drawback, as we keep repeating, is that that would be noteably more expensive than 2 cheap battery lights.

But on the topic of reliability, on the one hand I could get a flat tire, and I do carry patches, a spare tube, and a pump.

But on the other hand, there's no way to 100% eliminate possible failures. Your frame could break while riding - like I said it happened to me. It would be pretty amusing to always carry a spare frame on your back just in case. Your wheel could get smashed badly enough in the rack (purposefully or accidentally) that you'd need to bring a spare rime with you to fix it. Etc.

It's a matter of how much risk you think is worthwhile to take. I always carry my cell phone with me, and could call family, friends, or a cab, in case something like frame failure happened.

In the middle are things that you could carry a backup for but it's not worth the hassle. I could carry a spare chain, but I don't. I could carry spare spokes and spoke tools, but I don't. I could carry an extra tire (different than an extra tube), but I don't. I'd rather call a cab, or call someone for a ride, and then pay for that with money than go through the hassle of carrying that extra stuff - because it pretty much never happens. Fixing a tire I do myself, other more rare things aren't worth the hassle for me. It's the same with your car - any time you drive somewhere, there's a tiny chance your car will break.

On that list, a dynamo light is very reliable. Less likely, in my opinion, to break than a chain, spoke, etc. It's not impossible that it could happen, but like I said I've had a frame break as well.

The odds of something going wrong with a battery light are, in my opinion, higher. I've had several issues. Even then though, honestly, I've never been able to convince myself it's worth the hassle of 2 lights for the kind of biking I do around town. I've always found some sort of workaround that got me home. If the circumstances are more important or different than biking around town I'll bring a backup light - I always bring one mountain biking. It's pitch black, you're in the middle of the forest, you take a wrong turn and easily add an hour onto your ride and run out of battery, and crashing is far more common.

Sorry for the long post, what I meant to say is that I think a dynamo lights odds of failure are around the same as breaking an entire wheel, breaking a frame, or something like that. It's not 0, but it falls into the "use the emergency plan" area.

P.S. Remembered why I don't bring a backup light for changing flats - my cell has a led flashlight on it, and I always bring my cell phone.

I just haven't had that experience, not with decent commercial lights, since the advent of led's. There's no soldering needed for installing a dynamo light, and haven't had or heard of bulb issues since led's. It's not impossible that a mount could break or a wire could go bad, but I could break spokes on my wheel to like I was saying. Dynamo reliability in my opinion (if you're not doing anything custom) is as good as the other reliable stuff on my bike.

Sorry to hear that about your Gilligan Island experience...kinda want to point out a tiny bit that it wouldn't have been a problem if you had dynamo. But seriously, I remember the first time I went mountain biking on a real trail at night. I thought I had plenty of battery life, then my light went dim, then it went off. It was getting colder, you can't see a thing in the middle of the woods, no idea which way was even back without a light to see the trail, and since you're in the woods it's pitch black - no ambient light. Fortunately for me, in my paranoia, I had in fact brought a second set of batteries. I switched the connection over to them and went on my way. It wasn't easy to do in the dark, but I figured it out and things worked out. 14 hours in the middle of the night - wow, that's a long time.

Interesting. I definitely prefer a wider beam for the bars, but if I'm using a helmet light a narrower beam for the helmet.

You completely jumped over what I said before though. I find it more work and less useful to try to read to much into an OP's situation. I assume that if I say the advantages and disadvantages, they can sort them out without me asking repeated questions over and over again. I find that it's easier and best for both of us - posters tend to get annoyed with a ton of questions, sometimes posters will even say they want one thing then change their mind later in the thread, and even if I'm wrong sometimes it's useful to them later when they're thinking about their next move.

You'd have to define "casual user" - like I said, I personally own an $800 road bike and $300 in dynamo lighting on that bike. It's totally worth it for me, I never would have found out about it if someone had started making assumptions for me about what I did and didn't want.

I guess I'd rather not get into a debate on another topic.

I'm happy to discuss most things on a logical and useful level, and I hope other people are as well.