I've been using F# for a while. The main benefit I've had from having access to the source is being able to figure out exactly how things work. A _lot_ of F#'s "magic" is in libraries written in F#, and the source to those is very clear and easy to navigate and figure out. The compiler is of course more complex, but the few times I've wanted to figure out a specific issue in the compiler that the F# language spec didn't explain, it's come in handy.

This also holds true for the .NET Framework class libraries. Visual Studio can automatically step into the source while debugging. This has saved me tons of time.

When Microsoft decides to no long work on F# anymore, somebody else can continue to work on the project.

As it happens right now with IronPython / IronRuby.

I.e. it is just a promise that even if F# isn't receiving the same support as C# is, you have a safety net (kind of).

Value comes from the ability to tweak or change the language if necessary, for your application. A permissive license means you're less locked into the vendor (Microsoft).