Is there a link between the type of rider and the attire here?
Seems to be that those folk who do a lot of racing/cross/road riding seem to go to tights, whilst the more 'weekend warrior' folk stick with shorts. Perhaps I'm wrong.
I remember an article by either Andy Pruitt or Chris Carmichael saying that shorts should stay away under 60 degrees F, ie just over 15 degrees. I am often pretty surprised how the pros will keep their legs covered in pretty much all weathers!
I rather support bent_udder, there may not be a link between cold knees and subsequent damage, but keeping them warm is certainly not going to damage them!