One possibly corner-case example: Interlocked.Increment. Without passing the variable by reference, there's no way of performing the increment atomically.
I can't say I've used ref very much myself, to be honest - I generally steer clear of the need to return multiple values, and even then out is usually sufficient. Many of the cases where I've seen ref used, it's been due to the author not understanding how arguments are passed in .NET when it comes to reference types.
