I think it's nice that Microsoft is trying to get more people to get an anti-virus program. I doubt it'll be as good as aVast! or AVG, but, to me, it looks like they're just trying to give people who don't know about AVG, aVast!, or etc. an Anti-virus.
Personally, I think it may be a good idea to bundle that with an OS, provided they'll allow it to be disabled and/or uninstalled, and another AV to be installed. That should at least help prevent some viruses for people with no AV whatsoever.
"Don't fix what isn't broken; Just upgrade it."