Oh they absolutely would. Heck, Windows Defender is the same engine as MSE, they just had to limit its functionality to keep from getting sued. They were even threatened by the third-party antivirus companies when they promoted a link to MSE in Microsoft Update.

But hopefully they can take a more proactive course now that the DoJ's oversight is ending:

http://arstechnica.com/microsoft/news/2011/04/department-of-...