Imagine one is long, say 30 ES (that's ~$1,650,000.00 worth of S&P futures) and the Bad Guys nuke New York and Chicago, and Globex halts for 4 weeks. It finally reopens down 500 points (a tad less than -50%). That's a loss of $750,000.00 AKA three quarters of a million.
Now the daily margin...