I wouldn't call a former olympian with a bad back a monkey, he knew what he was doing and the mainsail took it nicely. No problem.
It would be interesting to hear Glenn's take on the same issue today as there have been quite some time since he said to always let off the downhaul when the sheet was released. Have other sailmakers expressed the same concern about the sails ability to take the loads from a full downhaul and no sheet?

When tacking in "normal" conditions I let out about 60cm on a 1:9 purchase system (I guesstimate, have never measured this). That should translate to a vertical movement of the boom for about 6cm? I dont know how much straighter the mast will go with a 6cm movement, but I think the bending forces you let out of the mainsheet system will have to go somewhere besides the leech. In the super light stuff we crank on the downhaul a lot to flatten the sail and get whatever airflow there might be to stream over the mainsail. At the same time we dont sheet much before we have accelerated. This should imply that the luff takes most of the loads bending the mast as we know that it's mast bend that flattens modern sails, not cloth stretch. Q.e.d. the luff is tougher than we think. Then there is the age old difference between shaving and cutting of your head.. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />
PS: We dont sheet out just to let the air accelerate faster over the sail after the tack, but also to shorten the time spent before getting over on the new tack. With a tight leech the main acts like a weathervane working against your helm. Same when going into the tack, sheeting in to help your rudders heading up into the wind (you know this of course, but for the benefit of those who might not know..).