Primes are great when...
Prime lenses are great. Normally, they are usually lighter in weight, often-sharper and normally have wider apertures than many zoom lenses. The only downside is that they do not zoom.
In the many-many years in which no self-respecting photographer would touch a still-camera zoom lens, many professional photographers using 35mm and just about all photojournalists used multiple bodies in order to increase the versatility of their prime lenses.
Even though, I have several zoom lenses, I am a great proponent of the multiple camera body system. I believe that using more than one camera is tantamount to being essential when using primes and is still very handy when using zooms.
Switching lenses is simple and efficient when you are carrying two or more bodies. While you do not quite have the versatility of a zoom lens; using two or more bodies does free you from being locked into one focal length without the bother of switching lenses.
When using Leica M-2 film cameras, my favorite combination for general shooting was the 35mm on one body and 90mm on another. That combo was great for up close people shooting such as at a wedding reception (I never used 35mm equipment for the ceremony or portrait shots but, did use 35mm for wedding candids).
If a person was to use a 200mm on one body and a 100mm on the other, he or she would be pretty well set up for many shots requiring tele capability.
I have a 30D and a 350D. Between them, they are probably worth somewhere around the price of a 40D. However, I would much rather shoot with these two bodies than to be restricted to only one body and one lens at a time. IMO, these two bodies, set up with two great lenses, are more capable than a single 40D would be; even with the availability of those two lenses. Sure, I'd like a pair of 40D cameras, or even a pair of 30D's, but my two cameras do a great job!
My standard set-up these days is the two bodies with 17-55mm f/2.8 IS and 70-200mm f/4L IS lenses attached. I carry one body on a strap around my neck and the other body with a hand strap attached, ready to shoot in a holster case at my left side. With these two lenses, I have a great focal range with awesome image quality. I don't miss the gap between 55mm-70mm a bit!