I understand what you are saying Mark and in larger games I have absolutely no problem with playing against as many Knights/wraithknights as you want to field, same goes for fliers. In the 500 point games I have played which did allow fliers it was in all honesty a waste of time playing (Necrons can have 2-3 fliers in 500 points, as you well know).
With the old knight codex it would have been a little more reasonable to run them at 500 points as you would have been only able to spend 375 of your 500 points and only had one ranged weapon. Now the new codex has dropped you can essentially remove a unit a turn and at 500 points means the game is over by turn 3, unless as Ray has said you specifically tailor a list to kill a Knight.
It's tough to balance, I love the Imperial Knights (I have one) and would happily play your all knight list at 1500 points or above.