The amount of wear is related to the amount of energy you pump through the slide. Think of how many square inches a 6x9 slide is, and how much energy you would need to make a sheet of that size look 'white'. Now think of how many square inches a 5 foot high projector screen is, and how much extra energy it takes to bring that up to looking 'white'... Even if you are jacking the energy per square inch up to overcome the other lights in the room you're still at a fraction of the overall energy you would normally use for a projector at a decent size.
If you aren't putting through enough light to noticeably heat the slide, and using a source with little to no UV, then you aren't really doing much more than leaving them out on your desk.
I've been toying with the idea of doing displays like these, and plan to use duplicates rather than the originals, but honestly my worry is more of them scratching, collecting grit/dust, or otherwise taking accidental damage than I am over the wear from the light source.