We're talking about something that prevents form data loss. If it doesn't error on IE6/7 and simply doesn't work, then that's fairly graceful degradation that provides a benefit to users on more modern browsers.
But I suppose if your userbase is predominantly IE6/7, then it may not be worth the effort for you.
At least in the federal government in the DC area, IE6 and 7 are all but dead, basically. While there are still plenty of legacy servers running older versions, desktops are far less behind than they were even 2 years ago.
I'm admittedly only speaking for a dozen agencies or so, but things are definitely better off lately than they used to be.