Sam Weber sent a note referencing James Gurney's blog, and in particular a post about his thoughts on using reference in the service of illustration and art. It's worth reading, and puts the issue in historical context that sheds light on the ways our forebears used the newfangled camera in their art.
For you newbies out there, James Gurney is the artist who wrote and illustrated the masterful series of books "Dinotopia". He spends countless hours making maquettes, studying various lighting schemes, doing hundreds of sketches and studies before he's ready to begin the actual painting, because his work is realistic fantasy and requires the highest level of believability. He understands the value and necessity for reference, because he spends so much of his time preparing it.
Here is the post on Photo Reference.