In before "it is Microsoft's fault", "copied Google" and possibly a "buy Yahoo!".

Malicious software to my computer isn't my major concern because a patched OS is fairly immune. I do know why they do this, but I am more impressed with phishing filters. On that note, these things use blacklist-style-security, which is inherently flawed. They do reduce some risk for the unalert and the unpatched, but will never keep you completely safe. Still, I suppose it does add a layer of security, and since the search results are of a known quantity that Microsoft itself lists they could scan all of them to a point. Just don't become complacent.