Generally speaking, we think of a "one-lens" solution as being a zoom lens which covers a wide focal range. The logic behind this is infallible: I can snap a wide-angle shot one minute, then wheel around and zoom way in on something else the next. And I never have to switch lenses (switching lenses is no big deal — but carrying them around is). 

And I can dig the logic behind that, too, and I often roll out with only my 28-135mm IS lens for a day of shooting. But just as often, I'll just use my 28mm f/1.8 lens and leave the zoom home. Why sacrifice the convenience of the zoom? The reasons are three: smaller, lighter, and faster.

Smaller. My 28-135mm lens is 4.25" long at 28mm and nearly 6" long when extended. The 28mm lens protrudes 2.5" or so. The Canon 18-200 IS (an excellent lens for non-full frame cameras) is similar in size to the 28-135mm, and covers a wider range of focal lengths. But I find my camera is better balanced with just a small "prime" (non-zoom) lens attached than it is with a zoom. It just feels more comfortable. Plus any zoom lens, over time, develops "zoom creep" from lots of walking around — it gets loose and tends to settle at its longest length. Some lenses have a handy little switch to lock them shut to prevent creep, but then you have to fiddle with a switch all day, and believe me, you'll miss a shot or two because of it. 

Lighter. My 28-135mm lens weighs more than a pound. My 28/1.8 weighs 6.5 ounces. Easier to carry.

Canon 40D with 28-135mm lens (left), and 28mm lens (right).

Faster. "Fast" lenses start at f/1.8 and go down from there. F-stop is a ratio of focal length to the width of the lens' aperture, so the lower the number, the wider the aperture can open (think of the iris in your eye). And the wider the aperture, the more light gets to the image sensor (or to the film, for all you holdouts). Photographers call a lens that lets in more light "faster" than one that lets in less light (slower). In daylight, this is not an issue. But as daylight fades, with a faster lens you can open up that aperture, let in more light, and get a crisper shot with a faster shutter speed. Image stabilization helps, but it only lets you hand-hold shots at lower shutter speeds — it doesn't let in more light or slow the world down for you. At some point, you need a "fast" lens with a wider aperture to get the shot you want.

With a prime lens attached, I find myself getting closer to photo subjects.

Okay, 3-1/2 reasons. "Prime" (non zoom) lenses, in most circumstances, give you better image quality than zooms. The reason is that they only have to do one thing well, whereas zoom lenses need to cover a range of focal lengths and inevitably make some compromises when it comes to image quality.

But what about the convenience factor of having a superzoom lens? Again, it's hard to argue against it. But I find a strange phenomenon happens when I have only my 28mm lens with me — I start to see the world through the 28mm focal length (okay, it looks like 45mm in my Canon camera). What I mean is my brain starts framing my vision as though I was looking through the viewfinder and blocks out everything wider and tighter, letting me tailor the day's shooting to the equipment I am carrying.

I spotted this shot from afar, and walked up to it until
the framing seemed right.

So with only the prime lens attached, I have to "zoom with my feet" (walk) when I spot something in the distance worth checking out. But like a golf shot, I believe there is a benefit of walking up to a photograph — makes me think about it a little. How do I frame it? Where's the light coming from? What do I want in the background? My goal is never to crop (or very minimally) in any shot I take.

I liked the blue crevasse here, but I didn't want it centered in
the frame. It's another case where walking up to the shot aided in its composition.

Crutchfield has quietly begun carrying a nice collection of prime lenses. I can vouch for the Canon lenses personally, because I've used them all at one point or another. But Nikon and Sony make great lenses, too. I really believe you can't go wrong with prime lenses. And if you have children, you need at least one fast lens so you can snap photos of them indoors, in twilight, or performing in the school play — no zoom lens is fast enough to shoot in the dimmest light.

Prime lenses are usually "faster," so you can shoot them indoors without a flash.

Zooms and primes both have their place in my bag. One is not necessarily better than the other, because each can do something the other cannot. But if you're looking to travel light or take pictures indoors, a small, fixed-focal length "prime" lens is a tempting choice.