The no-planes-at-all subset of the 9/11 conspiracy theorists are by definition either mentally retarded or mentally ill. There are thousands of eye-witnesses (do you have any idea how many windows of occupied offices, schools, and apartments had a clear view of the upper floors of the WTC towers?), who include many anti-Bush, anti-America leftists, who'd eagerly spill the beans if they'd been looking at the towers and not seen a plane hit (by the time the 2nd plane hit, virtually everybody with a view was staring at the towers -- we're talking many, many thousands of people here). There's also the matter of the staffs of the airlines whose planes were involved -- the mechanics, flight crews, air traffic controllers, etc. -- who all saw these planes take off and never saw them land. And never saw these specific planes again. Controllers all over the country had their eyes on radar screens for several hours afterwards, as all non-military air traffic was grounded -- no reports of spottings of stray planes that subsequently disappeared. And all those oddly consistent reports of cell phone calls from passengers on the planes, to still-living relatives.
If there were no planes involved, this would have to be not only a conspiracy on a historic scale, but also an extraordinarily democratic one, with participants spanning the entireties of the political, economic, religious, racial, ethnic, and age spectra. What could the motivation be, that would convince this vast number of very diverse people, who could not have been individually selected in advance, to participate steadfastly in a years-long conspiracy/cover-up?
But these are the sort of considerations which only trouble the minds of people with normal mental capacity . . .