Since the null-related blog post isn't published yet, anyone here knows the reason to implement null rather than write middle-layer in C# for whatever you want to use?
This change in F# makes it able to interoperate with C# NRTs, where C#'s T? becomes F#'s T | null and vice versa.
I feel like the ease of integration of F# into existing codebases is one of the major selling points. You don't need to write wrappers in order to consume most existing C# code, which, for example, enables writing a core with business logic in F# while keeping the IO in C# if necessary.
---EDIT---
Mean to post this under the GP