Radar can be a helpful tool for coaches in testing the endurance limits for a pitcher. Each year, we coaches need to bring pitchers up to game endurance levels from various states of readiness. Currently, I bet many of us try to use visual cues like are the pitches staying up in the zone, is he no longer following through or getting over the lower half, is he losing precision, does he look like he's laboring, etc and so on???
Some pitchers coast through bullpens at less than full intensity just to make their goal number for that bullpen.
Some work hard, but fall short of the goal number. It doesn't mean the bullpen was not a productive one.
I think that if a coach had an established mph range for each pitcher and each pitch, he could more easily gauge the effort level of the pitcher to make sure endurance / pitch count gains are real and not just the result of coasting.
I'm not suggesting that we show the pitcher their speed on every pitch or show them anything at all, but that the coach should monitor it by sampling a few pitches from each set during the bullpen just to verify the output of the pitcher.
Combined with the other visual cues, the coach can determine at what point in the workout the velocity / effectiveness began to drop off, he'd have a better understanding of the condition of the pitcher. I think better results would be achieved using the radar gun than to rely on the naked eye and your impressions of what is happening.