Get yourself into trouble, though, and MSE will poke out its head long enough to save your bacon. I downloaded several pieces of malware from the wild, including ones from Usenet news groups; MSE detected them as soon as they were removed from their zip or rar files. Its warning dialogs were clear, and the cleanup was quick and easy.
If MSE was a commercial product, it would have no chance of success. Most security software needs to be in your face to prove that it is doing its job, answering the "what have you done for me lately" question in a way that encourages users to renew the paid subscription. Even the free products like AVG need to find a way to make that model pay, so they have been doing annoying things like bundling Yahoo Toolbar with their products and popping up big windows that are little more than ads for the paid product.
Microsoft's motivation here is different, but still based on profit and just as urgent. The company needs to prove that those Apple parodies of Windows as a bumbling spyware-filled platform are way off base, and that Windows is still the best operating system. Microsoft Security Essentials has a very good chance of doing that.