Pete wrote:

Even in VMC, a traffic report may or may not
result in visual contact with the reported traffic, and ATC doesn't seem to
worry much about that.

In the northeast US, my experience differs. I have received climbs,
descents, and heading change instructions sooner after reporting visual
contact with reported traffic, whether the other aircraft were VFR or
IFR. While true that ATC may not worry whether I make contact or not,
I am pretty sure I am reducing their workload a bit when I do.

Just last Monday I was IFR and climbing out of my home airport to an
interim altitude assigned by ATC. Looking up ahead, I spotted a Cessna
Caravan at about 2,500 feet above me, crossing my flight path left to
right, that had not yet been announced by ATC. Thinking this might
have been the reason for my interim altitude (and hoping I could
prevent a level-off, which requires a lot of configuration change in
the Bonanza), I took it upon myself to call "unreported traffic at 11
o'clock in sight." ATC responded with, "Thank you. Maintain visual
separation with that aircraft, climb and maintain (my final filed
altitude)."

--
Peter