Update - Nov 18, 2008
Microsoft announced today that they will be discontinuing the OneCare product offering next year, replacing it with a free (albeit stripped-down) version code-named 'Morro'. Unfortunately, the part I liked the best about OneCare is being eliminated in Morro, the ability to know the status of the other computers in your circle.
Oh well, the search continues...
One word - time.
OK, maybe it's just because another birthday is looming, and it's occurred to me that I'm not getting any younger. If I were somehow able to add up all of the time I have invested in free anti-virus, anti-phishing, anti-trojan, anti-everything-under-the-sun types of programs I've used, and then add that to the time spent on free firewall, spyware and backup programs, and then multiply that by the number of computers in my house, well, it would add up to a lot of time.
And this is what Windows Live OneCare does better than any similar comprehensive protection/performance/backup program I've ever used.
It saves me time.