But I do wonder how many people actually compare performance with runners on as opposed to no runners on. 
Runners on is a stat, no runners on is not. So just because its listed in the pitchers split stats, I think it has importance.
I understand that you do not give one whit about velocity, but reality is, that its important to want to get as much velocity as one can from their FB because it affects all the other pitches that they throw. Although it doesn't mean that a pitcher that throws harder than another is better, its just the way it is, velocity at 85 or at 95 has its significance to each individual pitcher.
Again I think that it depends on the pitcher and what he feels comfortable with. I think that repeating the same mechanics over and over is so important so I really think that may be the thinking for sticking with one delivery over another. That doesn't mean that the pitcher will always pitch from that delivery, but I can see why the coach may want him to stay with the stretch, for now.
JMO