Claret,
I got the impression from your bolding of must that as soon as we got to 30nm we had to call visual, that was all. But you did actually mention the desire bit. Sorry! I was just pointing out that equally important is the "desire" first, THEN the "must " comes into play. I don't normally "desire" until I'm in real close!
Yes, the whole thing gets back to QF getting a visual approach at 30nm, not getting any more lower altitude assignments, and bleating about it. As I said, I don't think ATC are obligated to give further descents if the pilot has called Visual and has been cleared for a Visual approach. Obviously, the QF troops didn't realise that!
By all means call Visual at 30nm, but don't expect continued ATC descents. It's all then up to you. I for one certainly am not going to start pulling out my DME Arrival chart at 30nm: I'm gunna make ATC use that $600m dollar elephant ie TARTS to make my life easy. It is dark outside, after all...