The MS-PL license specifies: "If you distribute any portion of the software, you must retain all copyright, patent, trademark, and attribution notices that are present in the software." Also, source-code licensed under MS-PL can only be licensed under MS-PL.
That's going to throw a spanner in the works of anybody who wants to modify code and then pass it on under a different license, such as the GPL. Also, those who don't believe in patents or copyrights - never mind passing them downstream - won't like MS-PL.
Some see MS-PL as Microsoft's answer to GPL.
So, basically, if you develop a program that uses a program that is licensed under MS-PL then M$ owns the patents to it ? So they can charge royalty for it ? WTF ? Is this true ? Some open-source that is. M$'s version of open-source.