This happened yesterday and I missed it. Sorry about that.
It's an important update, as one can tell by reading the Apple advisory that accompanies it. The security flaws fixed here are both critical and actively being exploited, though there is disagreement on how widely. They affect both Mac and Windows users.
So, if you have Quicktime installed, patch it. It does not matter whether or not you use it. The application needs patching or you can be compromised. Period!