Virus definitions for MSE will continue to be provided until about June of 2015. It's the active malware detection that's lost support as of today. It will continue to function as an antivirus tool, but that's after-the-fact protection. It's upfront malware protection saves your butt most of the time. For example, if cryptolocker isn't stopped before it encrypts your files, running an anti-virus afterward won't do you any good. That's the real security problem with a compromised, non-supported MSE.
The new security software for Win 8 and 8.1 is a new *Windows Defender*, seems to work pretty good and is supposedly better than MSE.
Fundamentally, Windows Defender in Windows 8 & 8.1 IS MSE with a different name, so, other than it being fully-supported, there's no reasonable expectation that it would be vastly superior to MSE.
From what I read, it combined with the latest Win IE makes it a better anti-virus program than MSE...............According to Microsoft :)