The AV-Test results showing that MSE did not detect so many 0 day viruses have been a boon to antivirus vendors.
But in the real world, the situation is perhaps different.
MSE works on prioritizing the threats than might impact real world users.
Here is one article that explains the test results
and shows how many MSE users were impacted:
http://blogs.technet.com/b/mmpc/archive/2013/01/16/lessons-learned-from-the-latest-test-results.aspx
Considering that MSE is free, effective, and unobtrusive, it deserves consideration.