I'm not against Microsoft including anti-virus software with their OS; to me there's a difference between features (like IE) and protection (like, well not microsoft security essentials).
MSE doesn't currently have a positive impact on OS security, and it won't even if it's built in. Currently popular viruses aren't even detected by MSE and the ones that are usually aren't removable. Sure, it's better than McCafe but given the ready availability to users it's the first AV targeted by virii, and it isn't very well protected from rogue processes.
Even with improvements over XP, windows 7 is a basically insecure platform; UAC was a good idea, but it doesn't work and it's not enough alone. Code signing isn't a valid solution when "trusted" CA's are only in it for the money and they're in a market position where failure increases their potential future market. Lowering exploit counts and making a workable UAC would be time better spent by Microsoft.