Footed mainsail w/ flattening reef
You are picking on a pet peeve of mine!! I was "forced" to go to a loose foot on the last new mainsail (2000) that I bought. I must admit, shaping the lower one third of the mainsail, especially at the luff is much easier and better with the loose foot.
However, I am very uncomfortable with the lack of redundancy and the resulting mayhem which occurs with a blown outhaul or tack. Most outhauls were designed for horizontal forces only, not the entire sail tack loads. I have seen too many of these loose footed retrofits blow out in above average wind conditions, leaving the skipper at the whim of the headsail or auxiliary in nasty seaways.
As for AC cup use of shelf footed, my guess would be the design criteria is for a very restricted wind range. And with unlimited budgets, you can have the shelf cut in custom manner to act like a loose in reaches, and sail shaping rigging to take advantage of this cut while reaching. And, I believe we commoners have been encouraged economically by sail makers to abandon the bolt rope/slugs of the old shelf foots.