I fully understand that these aircraft are on an IFR flight plan but when they are in Visual Meteorological Conditions (VMC) it's the pilots who must look out for other aircraft and not run into them. Time and again I've had to remind controllers not to panic when I'm in VMC.
That may be the case in the USA, but whether you are IMC or VMC in the UK and pretty much the rest of the world if you are IFR you look at the instruments.
In fact I think that even in the USA even in VMC the requirement to look out for other traffic is only there once cleared to go visual by ATC. That is based on my limited experience of flying a very big jet into SFB, with all those light aircraft around I think that you would be off your rocker to accept a visual clearance in a 360 seater aeroplane!
None of the above detracts from the fact that the Captain is responsible and if he/she feels the need to look out of the window then he can.