John, anecdotally we started writing the open sill ends and were corrected by the local distributor. We had advised simply caulking them. He, informally, told us that was designed to be open, but offered no real scientific reason. Mentioned it may be for ventilation or prevent the water that may have penetrated the horizontal sill becoming seal into wood.
we still mention it. we have found, like you, many rotted and many nothing.
interested to see what others post.