It's a chlorofluorocarbon that's said to deplete the ozone. There are substitutes but I don't know if they are as effective as Halon. Fighting an ordinary fire in a high rise is a very difficult challenge. Fighting thousands of gallons of burning jet fuel hundreds of feet above the streets is even more challenging. Perhaps the buildings' internal firefighting water systems were still functional after the impacts (doubtful), but water isn't the best thing to put on burning kerosene. For that, you either need foam or something like Halon that can snuff out the oxygen supply. Retrofitting all the high rise buildings with those systems would cost billions.

At a gut level, I'd like to see New Yorkers rebuild in the face of this disaster. Chicago rebuilt after its great fire and San Francisco always rebuilds after an earthquake. We rebuild after hurricanes, tornadoes, and fires, so I hope we'll rebuild after this act of terrorism. Rebuilding is a re-affirmation of hope and a refusal to admit defeat. I hope they rebuild, but as Mary Anne said so well, it is a decision for New Yorkers to make. I'll fully support whatever decision they make. They've earned the right to make it without criticism from the peanut gallery.