It doesn't get a zero exactly, it says it's a "baseline". Sorry if I linked to a questionable source, but it's not the only place I've heard this from. AV Comparatives does the same (PDF) http://www.av-comparatives.org/wp-content/uploads/2014/10/av... Under the chart on page 3, it notes that MSE scored 80% (which is lower than the others) but it's not "competitive".
For Protection Score, it's rated 0.0/6.0. For something that's 80% accurate, included with every computer by default (and turned on by default) and constantly updated, it's being done a massive disservice. Yeah there are better free AVs out there, but given the choice between nothing and MSE/Defender, I'd really rather everyone use MSE. There's a long way between 80% and 100%, but an even longer way between 80% and 0%. Especially when the AV is no longer even the most important part of PC security. That starts at the browser (every browser) and continues into UAC (on every OS) and things like SmartScreen verifying the reputation of executables (not just scanning for malware).
I wasn't meaning to call you out on your source, just the message that this test seems to give off ("MSE is worse than nothing").
