LordHope hit the nail on the head. It's obviously the props. With out us, there is no scrum ie no scrum resets, ie a full 80 minutes of running, as apposed to the easy 60 minutes we sometimes privilege you.
On a serious note, all players have real importance. If your tight five is useless, then you'll always struggle, as the opposition will win all scrums and lineouts, which people under estimate just how important they are in a game. If your tight five competes well in them, it will limit the options of the 1st 5/8th, as will not be able plug the corners with kicks. I can see why people think the fly half is so important, for no other reason than he is the first receiver, so if he can't catch a ball, or pass well, it will limit the entire performance of the rest of his backline.