I never, ever fly without at least trying to get traffic advisories, and
it's very rare I don't get it. As a student, because NONE of my instructors
ever did, I didn't think to much about it (they are the pros, don't you
know?). Now, I consider anyone who is to lazy to get flight following as
someone too foolish to fly with.

Are there still instructors out there who still opt out of this (what I
consider mandatory) flying aid?

The facts are that: 1) you can't always get FF when you need it most
and 2) as many posters have pointed out, airplanes are very difficult
to see, especially behind you.

I fly in Chicago and have often used FF when available. But the most
common scenario is that the FF is abruptly terminated when I get close
to the Class C veil, which is when I most need it with a reliever
airport every 5 miles. This has been a major motivation for me to get
my IFR rating, which I use rain or shine. Even at that, I see maybe
half of the conflicts called to me.