Given that almost all scheduled airline ops operate on IFR flightplans which assume flight in permanent IMC how can a visual lookout be mandatory? Sure you'd use it where you can, but in IMC/IFR it cannot, ipso facto, be mandatory.
I've recently had an FO react in a most inappropriate manner due to a percieved conflict with co-ordinated IFR traffic that would have taken the controller very much by surprise had it continued. Sometimes acting on what you see by looking out of the window can be quite counterproductive.
Personally calls like "Bigbird xxx maintain heading due crossing traffic 10 o'clock same level call clear of traffic" make me cringe. How can we assume that the traffic we think we've identified is the one he intended? We cannot. Thus taking the authority for avoidance based on our own unverifiable visual criteria is surely a nonsense in the controlled IFR environment? Dosen't this negate the entire point of IFR altogether?
My answer to the above is invariably "not visual", as per our flight plan.
Lookout in IMC/IFR? How?
Incoming!!!