Good idea Jack, but it requires people to (a) know that they can't spell, and (b) care about it.Exactly. It won't stop people using "your" when they mean "you're", "there"instead of "their" and so on because they have no reason to check. Nothing alerts them that it's wrong.
I am not picking on anyone in particular, but I think that there a few people who might benefit from using Grammarly (http://www.grammarly.com/), it is free and available on all platforms P.C., Mac and Mobile Application (Android & iOS). As a Dyslexic, I have found it useful. I have been using it for more than Two Years, and have seen improvements.Name names Comrade!