Well I happen to have 2 single D lights and 2 single C lights as mentioned above and am very very happy with them. These are Arc LS bodies. I do not carry them on my person but are always close at hand. A set rides in my truck and set lives at home.
Where these are very practical is in the using up of batteries. I seem to always end up with odd batteries and I do not mix batteries. The makes single battery devices valuable to me.
Some people may say the the single 1.5 volt powered light is not as bright as a 3 volt one and thats true, but not by as much as you think.
Also, we seem to want to use more light than we really need.
If the single battery light is not bright enough, I have others that I can use but I find that 90% of my needs can be met by a single cell light.
AAA, AA, and 123 for on person carry. C's and D's for home and car.