Or, don't work well. Does nobody remember the lessons of the infamous F-111? It was going to replace fighters, attack planes, light bombers, nuclear strike bombers, for both the Air Force and the Navy. The plane went WAY over budget, and in the end, the the F-111 turned out to be a pretty good light attack/recon aircraft, but not much else. http://en.wikipedia.org/wiki/F... Instead of a plane that's excellent at say, dogfighting, you get one that is mediocre at dogfighting. And VTOL/STOL. And attack. It's like using a leatherman when you could be using actual tools.