As most people mentioned, lithium powered lights have an advantage over regular AA, C or D in terms of shelf life and performance. If you were going to store a light away for any length of time, I wouldn't trust regular alkalines, especially if the temperature varies widely (like in a car).

For high power lights, lithiums are the only way to go. Alkalines just can't supply the energy needed for high performance in a small package. Recharables are an option, but the shelf life of those are even worse than alkalines.

If you're worried about longer term emergencies, I still think the advantages of lithiums are worth it. Most likely, for longer emergencies over 5 days, you won't need really powerful, bright lights. Simple low powered LED lights should be sufficient in most cases, and most any of the LED lights should have enough battery life to get your through it if you use it sparingly. Plus, if you're in that kind of situation where you're without electricity for that long, I wouldn't rely on which battery is most available, most likely ALL batteries will be gone. So it really doesn't matter what you have, if you dont' have it at the beginning, you won't be able to buy it. I do keep a few AA maglites w/ LED conversions around, but that mostly because I know I can scavange batteries from around the house if I need to. Most all of my other lights that I use are CR123 powered because I buy them in bulk and store it, and it actually a lot cheaper than using lithium AA's.

Actually, I have lights for almost every type and size of battery available, so I'm not too worried about finding the right battery, I will always have something that can be used. But unless you want to start up a huge collection of flashlights (very addicting) then maybe that's not an option for you.