I guess that's useful for people who are already bought in to the .NET world, but isn't LINQ encumbered by patents that pose a risk to anyone building on top of it?
You have always been able to build anything on top of it without concerns about patents from the implementation. This claim was so hilarious that it is incomprehensible that anyone ever believed it.
You have also been able to reimplement .NET CLR and core libraries without fear of patent litigation (from Microsoft). They have placed the CLR and core libraries under the legal estoppel of the community promise since 2007 (IIRC), in addition to publicly granting patent license to anyone creating an implementation of NET CLR and core libraries from the specifications. This latter was part of the process under which C#, .NET CLR and core libraries was standardized under ISO. A precondition for the standardization was that any necessary patents (for implementation) be offered on RAND terms (reasonable and non-discriminatory). Microsoft has always offered the patent grants free.
The community promise was created in response to FUD from (among others) FSF that Microsoft would just sue anyway (despite patent grants), and with their vast army of lawyers and deep coffers they could bury in court. The community promise creates legal estoppel, whereby a case by Microsoft would be dismissed if you acted "in good faith" by relying on the promise.
Open sourcing Parallel LINQ has no bearing on the patent status of anything building on top of it. If you believe the FSF FUD, you will be ensnared in .NET technology and Microsoft will sue you out of existence if you ever become successful. If you do not believe the FUD, you can continue to take advantage of LINQ, Parallel LINQ, .NET, C#, F# etc.
At that point, who cares?
For example, if your company had the same strategy as Google (and/or made some similar mistakes) with regard to circumventing Java licensing/restrictions, I could see investors or possible investors that were aware of that being somewhat spooked at certain points in the past.
Or did I misunderstand?
However, as far as I understand it, the patent grant only holds for .NET runtimes you write yourself or programs to be run in a .NET runtime.
https://github.com/dotnet/corefx/blob/master/PATENTS.TXT :
If you file, maintain, or voluntarily participate in any claim in a lawsuit alleging direct or contributory patent infringement by any Covered Code, or inducement of patent infringement by any Covered Code, then your rights under this promise will automatically terminate.
vs
https://github.com/facebook/react/blob/master/PATENTS :
The license granted hereunder will terminate, automatically and without notice, for anyone that makes any claim (including by filing any lawsuit, assertion or other action) alleging (a) direct, indirect, or contributory infringement or inducement to infringe any patent: (i) by Facebook or any of its subsidiaries or affiliates, whether or not such claim is related to the Software, (ii) by any party if such claim arises in whole or in part from any software, product or service of Facebook or any of its subsidiaries or affiliates, whether or not such claim is related to the Software, or (iii) by any party relating to the Software; or (b) that any right in any patent claim of Facebook is invalid or unenforceable.
-Not directing anything at you specifically
According to the wiki (http://en.wikipedia.org/wiki/MIT_License),
> Whether or not a court might imply a patent grant under the MIT license therefore remains an open question.
This probably means MSFT is unlikely to enforce those patents.