So, there's the 'know you're going to need it because you use it all the time' school, and the 'just in case' school.
I carry an Inova Micro or ArcAAA on my keychain for 'just in case'. As it turns out, these little lights, while very useful, don't put out enough light for many of my needs, but more than enough in case things go dark unexpectedly.
I live in an urban environment, and when I need a light, it's usually to look into a deep shadow created by a streetlight.
The little lights just don't can't compete to light up an area when your eyes are adjusted for the bright streetlight level. Great example is trying to change a tire when you're on the shadow side of the car.
Same thing when looking under the desk for a computer port - I need a light that will let me look into a dark hole from a brightly lit exterior. Same for looking in my kid's ears or noses to see if there really is an Altoid stuck in there. Same for looking at things to see if there are cracks.
The Arc LS series does a fine job of this, and is my 'use it every day' light - at least during the winter, when there are many hours a day of dark here in Seattle. In the summer, I hardly bother, as it's only dark from 10pm to 4:30am or so. In the dark of winter, I use some light daily, and need one about once a week.