I confess that I find it quite comforting when I'm bouncing around in cloud, there's an airliner somewhere above or below me, and they report "we have him on TCAS" - as at least one of us knows the relative position of each other from a threat perspective.
Regarding driving a head into the cockpit, I think we should in that context remember just how large the blind spots are in most civil and transport cockpits.
I'm afraid that I don't have any diagrams for modern combat aeroplanes, but here's a Hammer diagram (this presents a 360degx360deg view onto a flat image, like a wall map of the world) for the Jaguar front seat...
There are blind spots, primarily below - but not that many.
Now compare to a Boeing 737 from the Captain's seat...
There are a lot of blind spots - including even in the pillars which unless they are less than the distance between the pilots eyeballs (typically around 80-90mm) are also a significant obstruction. Plus that it's asymmetric - so if the Captain is the one doing the lookout, she has a lot of blind spots to her right that she can do nothing about whilst her F/O is busy heads in.
And most light GA are only marginally better - this is from the left hand seat of a PA28...
It's really not hard to see huge blind spots there, and equally there are coincident blind spots - a descending or climbing PA28 has plenty of areas in the direction of travel they can't see.
So, I'm afraid I don't buy the argument that a small and well managed period of "heads in", in virtually any cockpit, isn't entirely well spent, if that is spent for nearby traffic on a screen - whether that screen is providing pure alerts, or spacial awareness. Clearly excessive such time is not advantageous but when the aeroplane structure is obscuring perhaps 60-70% of the outside view, spending 5% of one's time (these are numbers plucked from the air with no science behind them, so likely to be the wrong numbers, but you get the idea - 5% however is 10 seconds every three minutes) checking an electronic device for external threads does not seem to me to be excessive.
G