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.
Sure you can quote stats all day, but it depends on your target audience. If you are running a tech website (such as HN/TC/Reddit/etc) you will see a HUGE portion of visitors using browsers like Chrome/Firefox and hardly any IE.
However if you are building websites that regular people use they still use XP and IE7... so much so that it can account for 10-30% of your website traffic.. It really pains me when HTML frameworks like Foundation 3.x drop support for IE7 and below because there is still the userbase there.
IE7/8 is not dead for most of the world yet. People who run statcounter websites are getting data from people who run their tools. I would trust if someone like Google Analytics released an average browser usage as they are more widely used. Who the F uses statcounter nowadays?
 Though you can do tricks/hacks to make it work for IE7.