The conclusion is that while most tools had a detection rate of 100% or slightly below that, Microsoft's Security Essentials tool scored horrendous with a detection rate of only 73 percent on real-world testing and 87 percent on the real-time test. The poor detection rate of Microsoft's Defender/MSE security products is not new but these tools are popular as they're available for free.
On one hand a poorly performing anti-virus solution that is installed by default is definitely better than no protection at all, but on the other hand it provides a false sense of security as so many pieces of malware slip through.
You get what you pay for. As Erik Heyland, director of the AV Test Testing Labs, noted, "An economical option for protecting a corporate network is the use of the endpoint module, bundled in the Microsoft Management Suite System Center 2012. The test revealed, however, that this is not to be recommended. The solution was awarded 0 points by the testers in terms of its protection function, and it achieved only 11 out of 18 possible points."
His final conclusion: "The freeware Microsoft module is much too feeble in the detection of malware."