As admiral Ackbar said once...
ITS A TRAP!
Microsoft has cracked open .NET a little further and surrendered some control over its development platform to the open-source community. The latest code for the company's take on Python and Ruby – fine-tuned for .NET, IronPython, and IronRuby – has been quietly released under the Apache Software Foundation (ASF) license. So …
So is MS' version of Ruby written from scratch then? I assume so otherwise they wouldn't be able to relicense it from GPL to ASF.
That being the case (ie - it's a rewrite rather than just a tweak of the official version), I can see why the take-up would be very low - it's always going to lag behind the official version and there's always going to be issues of compatibility.