I take photographs for my own use, but I want them to be as good as I can make them. I'm willing to invest in lenses, but my experience has been that 70-80% of the shots I take are created pretty quickly: grab the camera, adjust the zoom, and shoot. I often adjust the exposure for DOF, or to be where I want with the lens' aperture, but I don't have a lot of time to set up each shot.
When I was using 35mm film (with 80s glass), my standard setup was a 50mm prime and a 75-300mm zoom. I kept the 50mm on the camera most of the time, and only put on the zoom when something was so far away that I expected to see objectionable grain in a blow-up.
I now have a Canon 500D, and a 17-55 2.8 IS USM lens, and it's pretty much always on the camera. The difference between that quality zoom lens and a prime isn't enough to make me think a prime makes sense, but that's wrapped up in this whole question. And I do carry the 75-300 zoom, and get very good results from it: I can't complain about value for a lens I bought 20 years ago or so.
I guess my question probably boils down to this: does a professional (or experienced amateur) photographer get so used to switching lenses that they do it as a less experienced person adjusts exposure? Or is it more that a professional photographer is in situations where the expectation is to get it right, whatever that takes, including setting up lights, switching lenses, adjusting the scene?
I don't suppose there's a single answer to the question, and different fields (like photojournalism) probably approach it differently. But I'd love to know how the serious photographers here approach the issue.