Of course, anyone who thinks they can "design-around" a patent will claim that the patent is not actually "necessary" to the desired function. In order to enjoy this "promise" you have to confess that the only way to achieve the standard is to infringe on a valid MS patent.

Another potentially worrying point is this exception:

So..., anyone who even "participates" in a patent suit against MS (including, presumably, a patent suit filed by MS), loses protection for not only their own products, but anything they "use." For example, if FSF got into a dust-up with MS, MS could still claim infringement by FSF (or anyone else) for using Mono, even though MS has chosen not to pursue the authors of Mono or other Mono users. Referring to it as a "personal promise" also calls into question its applicability to businesses/organizations.

I am being overly paranoid, but the fact is, like so many other legal "promises", the value of this one will only be seen in the implementation. As a cynical lawyer, I don't see anything here that absolutely precludes MS from asserting infringement by Mono or any other OSS project. If Microsoft truly wanted to be benevolent they could easily make a much broader promise with less grey area. For example, they could name the patents that they claim to cover the specs in question and offer royalty free licensing, or make a non-assert pledge with some actual teeth. All they are saying here is "we probably, maybe won't sue you unless we do."