Why out ref will be an issue

gistfile1.fs

namespace Test

type MyType = 
    static member AsyncWithRefParams(i : int byref) = 
        async {
            i <- 5
        }

module Program = 
    [<EntryPoint>]
    let main _ = 
        let result = ref 0
        MyType.AsyncWithRefParams(result) |> Async.RunSynchronously
        printfn "Result: %A" result.Value
        0