Deja vu all over again ...
Does anyone remember the last time Microsoft came out with a free antivirus system? I think it was back in the DOS 6 days (give or take some), and was quickly found to be pretty vulnerable (word was that you could disable it with a couple of assembly-code instructions).
Even if this one isn't badly-designed and/or badly-implemented (which would be a first for Microsoft on a V1.0 product), the danger is that, being from Microsoft (and therefore good in the eyes of the Great Unwashed), it could become the de facto standard. And that's dangerous.
I believe that, bad as the malware-on-Windows situation is, it would be worse if everybody ran the same AV program. Microsoft's AV team misses a 0-day attack? Great, the whole world catches it. An update crashes the system? Everybody crashes.
There's a certain hybrid vigour to having lots of different AV programs. If one of them misses something, the odds are that at least some of the others don't, and some of us will still be safe.
Now, I don't know if the other AV vendors are arguing along these lines, or are just bitching because they see the gravy train departing at high speed (I wouldn't trust Trend Micro as far as I could throw them, for a start, after the Barracuda debacle). This time, though, I agree with them.