Perhaps the first counter-question to aks would be: have you ever really needed Microsoft support for a tough problem? Did they actually contribute to a solution or did they simply take you by the hand and walk you through the obvious checks and possible fixes that you had already tried yourself? Did you sue them because the problem wasn't fixed and how did that go?
But regardless of that: what if Microsoft started selling, say, hard disks (like they sell mice and keyboards), would they be used to the exclusion of all others? What applies to software should apply to hardware as well, right? Because isn't hardware just as likely to contribute to data corruption, finger pointing and other issues as software, if not more so? So following the same line of argument one should us these Microsoft disks, regardless of how such disks might perform compared to others. And regardless of their price, capacity, life expectancy, suitability to be used in arrays etc. This example may not convince them (e.g. because Microsoft isn't selling hard disks yet), but it certainly shows the stupidity of the argument that 100% Microsoft is always better.