Cris is right, of course, except when your more expensive lens is too damn heavy to drag around, so you leave it at home and don't have it when you really wish you did.

Perhaps more seriously, it would be interesting to do a study where you put the very same lenses in front of film and a full-frame digital sensor, run the film through some super-duper scanner, and then make your comparison. The assertion, for example, that bokeh is different is easily testable.