It makes sense. Almost every target of every virus, worm, trojan - is Microsoft software. And Microsoft is pretty good at knowing what part of their software is being attacked. Other antivirus companies have to wait for MS to tell them when there's a problem - not so if MS is doing the protecting themselves.
And did I mention it's free? Of course there would be a HUGE uproar if Microsoft charged $$$ for protection from problems that they should have foreseen. It would also look like a terrible temptation for abuse - break it so you can be paid to fix it - and I'm sure Microsoft is not interested in developing that kind of reputation.
I've been using Windows Security Essentials since the middle of December, and I'm very happy with it. It gets good reviews, and it's not bogging down my system. What more could anyone want?
* Speed up a slow computer * Clean malware infections * Install new programs or accessories * Repair system errors * Consulting and advice * Solve connection problems * Tame the computer beast