I use primes only for my Canon FD bodies.
I use zooms on my Pentax MF and Canon EOS bodies.
When backpacking a Canon Rebel 2000, 20-35 USM and a 28-105 USM Zooms cover everything I need and are much lighter than primes.
If it's just a walk I'll take anything that takes my fancy that day from a Canon FTBn with a matching BL 50/1.8, or a T-90 with a handful of primes, or an EOS-3 with zooms and primes. Or maybe a Mamiya 645 with primes. It just depends.
Sometimes the choice of tool (and a camera is just a tool) is driven by terrain and accessability, other times the creative vision (pitiful though that may be in my case) is the driver.
Galen Rowell had a lot to say on matters like this and I tend to share his approach to equipment, and always have.