Yeah this is the more practical problem than legality. Jamming the mainsheet might be ok on a light day with you close hauled on a single tack to windward over a long distance e.g. not racing. But as soon as hiking weather begins, jamming the sheet is the last thing you want. You'll find that out pretty soon. (Happy swimming!That answers it definitively. Many thanks. Now I'll just have to work out how to uncleat it mighty quick when I'm too slow hiking out on the new tack!
... But the central jaming cleat is one way to help you free one hand to do some other task, for example trim the wang...