ByRef extension methods would be very cryptic, since you do not expect c.DoSomething() to replace c.
IValueConverter solves Visiblity problem in WPF, I am not sure if Silverlight supports them, but if it does, it is just a minor annoyance, not a PITA.

Anyway you can 'ByRef' properties by doing ExtensionMethods.SetVisible(value => IsPanelVisible = value, true), which is a bit longer but works fine.

This and a previous post are the least performant and recommended ways to do OOP in JS.
The correct way is to use prototypes.

Even the IE team recommends it:
http://blogs.msdn.com/ie/archive/2007/01/04/ie-jscript-performance-recommendations-part-3-javascript-code-inefficiencies.aspx

I appreciate author's effort, but it is a pity a bad practice got it to the front page.

Following your point, why not a whole solution in single laguage?

There are often XML, XSLT, ASP.NET markup, Javascript, CSS and C# living together in the same project.
And things like Regular Expressions live happily even inside the C# code.
Also, the projects themselves are MSBuild.

Does it really makes everything harder to understand?
I doubt that Xml transform in C# (without XLinq) is more understandable than Xslt.
And if I write Xml transformation in Xslt, isn't it better to write tree walking in F# active patterns?

Now what's missing is the ability to write a part of a partial class in F# and a part in C#.
Or, at least, an ability to keep different language classes in the same project.