I think the "problem" with the "Leeward boat has right of way" rule here is, the boat coming upwind (the leeward boat, or ROW) has the best view of what's going on, as he's looking upwind in the first place and he's not got a big spinnaker blocking his view.

The 'burdened boat' is coming downwind with the spin up, blocking at least part of his downwind view, and the skipper is most likely looking upwind for a puff to chase, not always looking downwind for traffic.

If the ROW boat was at one time well clear, but then tacks, and the burdened boat never see's him tack, and now the ROW is in the spinnaker blind spot, well, it can get very interesting, very quickly.

I know when I'm the ROW boat, going upwind, I never assume the spin guy coming down sees me, and I'll gladly tack to get out of his way...more so if it's blowing and he's hauling butt. I don't want to end up "Dead Right".

I think it goes to the "avoid a collision at all costs" rule.