I pose this question because I often find that when I have framed the photo,

on my Canon 40D (1.6x factor) I see that I have selecetd around 50mm.

Not true for all shots, but it has left me wondering If I would not be better off with

2 or 3 fast primes, say 14mm, 50mm which I already have, and a 100mm

Then let my feet do the rest?

Zoom lenses are very versatile and very useful in situations where you can't predict which focal length you may need or when you can't walk to go closer. I find zoom very useful in urban/street photography. Of course zoom lenses tend to be less fast, less sharp, and not all are able to maintain the same aperture on all the focal range. So they have some more limitations and some advantage. Personally I prefer zoom lenses that include only tele focal ranges, I found them very versatile lenses and with higher quality results in terms of sharpness (tend to have less aberrations compared to zoom that include wide to tele focal length). If your need is sharpness and/or shallow depth of field and/or low iso in low light then probably you need prime lenses.

Although primes are generally superior in image quality it does really come back to what you are shooting.

I shoot female glamour for a living, primarily in nature where my feet cannot always do 'the walking'. A model in a rocky. difficult access, mountain gorge or frolicking in the surf is a prime example of where my zooms come into play so that the composition is correct. The only practical use I have for primes are in the studio.

It's good to have a combination of both.