IMO plexi is just too brittle. I suppose you could go way overboard on thickness, and get added toughness, but it's still not going to take repeated flexing very well.
Polycarb would hold up a lot better, but needs care (though I've been running mine for about 3 years without any noticeable degradation. I suspect sun will eventually do it in, well before scratching.)
I don't even like when guys bring glass bottles on board, much less having a glass windscreen.