You are right, but it doesn't feel the same as for example using freemarker instead of JSF or whatever. My point was more, if you start using some non-MS projects and you want to look up more information about how other people use it, that information might not be there or you will have a hard time finding it. If you are new to .net this can be a quite anoying. There are always exceptions for everything, but I wouldn't recommend someone new to .net diving straight into these alternative things.
This is completely false. There's a whole movement (http://altdotnet.org/) of developers not doing it Microsoft's way and getting along just fine.