I think the "rule" is a bunch of crap, considering so much of IDPA requires you to shoot from a kneeling position. Let's face it, most ranges are chat, gravel, and an assortment of spent brass. None are forgiving when you drop a kneecap on them.
I think they shoot be a little more concerned with shooter safety than "Is it tactically correct " or "Do you always wear knee pads when you go out".
Just watch the muzzle direction when a shooter drives his knee into a piece of brass on the ground with the casemouth up. Ever seen a cookie cutter?
Plus some of us have had prior knee problems and a little padding will go a long way in our ability to keep coming to these matches to participate.
Just wear one of the neoprene knee "braces". They don't really have the padding like regular kneepads, but they will help prevent injury to you and officials can't say much about a necessary piece of medical support. It would be like saying you can't wear hearing or eye protection because you wouldn't wear them everytime you go out either.
I have worn them when wearing jeans and when wearing shorts. If anyone complains, I tell them I have a bad knee. Which I do but a lot of it is to protect what knees I have left.