Just to clarify I believe there's a right time to throw hard and that is during the development process. That is in contrast to Midlo's funky off speed guy who never tries to throw hard. He's right that guy may never develop his fastball to it's max potential.
There are very few pitchers who can be successful pitching within 1 or 2 mph of their max velocity but practicing at times at those velocities is what can help a pitcher get near their genetic limit. There are different ways to get there. Extreme long toss can help some pitchers. Over/under balls can help some pitchers. Throwing hard off the mound in the pen can help some pitchers. Throwing hard in scrimmages can help some pitchers. It just comes natural to some pitchers and they don't need to do anything special to get to their velocity.
There are pitchers whose mechanics can be adjusted to get more velocity and there are pitchers who by trying to throw hard and getting feedback of some sort will automatically adjust their mechanics to do whatever is getting them that bit extra velocity.
Once you've got the velocity then most likely you are going to have to work to improve your command and movement. In most cases that is going to mean a working velocity 3 or 4 mph below the max velocity. I just think it is better to be throwing 93-94 and be working 3 to 4 mph below max than throwing 83-84 and working 3 to 4 mph below max.
As far as what TPM said about throwing 97 coming out of nowhere I'll certainly agree with that. I've gotten my son at 88 mph out of nowhere in games when he's working 83-85. On the other hand, that followed working on throwing hard where he was able to get up to 88 during a pen and figure out what he was doing differently to get there because he had feedback. He had control during the game but I doubt if he knew what he did to get more velocity on that one pitch. He had little control trying to hit a number on the gun in the pen but he did know what he did different when he threw harder.