nope I have a spec of dust inside my 300f4L and it causes no issuesIf a lens has some small dust internally it's nothing to walk away from I'm guessing?It theoretically reduces the contrast of the lens. Nothing you'll notice in your pictures. However, many people like the idea of having a clean product. If too much dust starts heaping up it might become noticable or make the moving parts in the lens move less smoothly, or in the worst case scenario, block the aperture curtains. Doesn't really happen much in real life though.
After reading this article I no longer fear dust. http://www.kurtmunger.com/dirty_lens_articleid35.html