Norman - the reality is that some American fighters have been gunless. Indeed during the Vietnam War some Phantoms missed out on kills because of the unreliability of their missiles and not having a cannon.
It's great to think that adding a gun to a Harrier would transform it but the truth is that it's extra weight and the Harrier is vunerable to ground fire.
Witness the losses in the Falklands War and low level in a hostile environment of is not where you want to be.