Today there was a really interesting post on Olaf and Kasia photography blog. They were kind enough to answer, in details, a question I asked in the comments a while ago regarding the change of lenses on the field and how to chose lenses before a trip.
As they explain, there is no magical and perfect solution. One should consider the lens as part of the guidelines for the photography of the moment and this is why using a prime lens allows to focus on the image itself. I can't agree more, having spend so much precious seconds hesitating on the focal length rather than taking the picture!
But the flip side of prime lenses is that the guideline they bring is also a limitation in some situations, which means sometime missing an opportunity for a good shot. Tough call... That's where having a second camera with you in important occasions allows to quickly switch focal lengths without having to fiddle with a lens or converter.
Anyway, I'd strongly recommend following Olaf and Kasia's blog, as they bring both great advises and good inspiration with their pictures.