Early J/24s like the late '70s one we once sailed came with a twin luff groove system. Sometime prior to the 1991 copy of their rules I have on my bookshelf, it became an option and a specification was added for the size and spacing of cloth hanks, although bronze hanks are also used particularly for the jib. J/24 Class rules also allow a jib Cunningham and require only one jib halyard.
My experience from serving as bowman on a J/24 in their Nationals a few years ago is it's easier to raise and lower the jib or Genoa. The sail stays on deck when you drop it so you don't need to go to the point to pull it down or feed it. Easing the halyard allows scallops in the luff as opposed to wrinkles with the a luff groove system. There is also the question of whether there is less weight aloft.
Even though this was overlooked or ignored during two measurements and your boat got a pass this year, I don't see how this could be construed as legal unless it has now become 'accepted practice'.
