3rd-party extensions have never been supported in the free Visual Studio Web Express. So you haven’t been able to install Web Essentials on Web Express before. With Visual Studio 2013 Update 2 and Web Essentials 2013 version 2.1 that is no longer the case.
You can now install and use Web Essentials on Web Express and get the exact same features as you would on the paid versions of Visual Studio. It’s the same code that runs in both. In fact, it’s the exact same extension that runs in both.
The reason for opening up Web Express for Web Essentials is to be able to provide cutting edge web tooling for free to anyone interested. Since Web Essentials is the ASP.NET and Web Tools Team’s official unofficial playground, we thought it would make perfect sense to broaden the support to the free versions of Visual Studio.