I have several SLRs without instant return mirrors, and in my experience not having an automatic aperture is a far bigger hindrance to capturing a shot than not having an instant return mirror. Other's experience may differ, but one could also point out that many manufacturers considered the automatic aperture more important than the instant return mirror too.
If you're shooting nature for instance, and a bird pops out of a tree, you're going to be pretty lucky to stop down a preset lens and get your shot. You cannot leave the lens stopped down either if you need to adjust focus quickly. For candid situations the automatic diaphragm is basically a necessity, an instant return mirror is simply a convenience.