Being a man of clarity and precision, the instructor explained to us that we should never release a pin just because someone taps."
Could you clarify this part? It reads as if the instructor knew the free arm was pinned under the body. It also reads as if uke was hurt enough to not continue training the rest of the weekend. Is that correct?
FWIW, IMO, if you're demonstrating, there should be absolutely no need to over exert any pin to the point of hurting someone. Pushing limits is one thing, hurting is another. And if a "highly ranked" instructor can't pin someone effectively in a training environment or demonstration without hurting them, they shouldn't be "highly ranked" at all.
The entire reason the "tap" is there is to alert nage/tori/instructor/whatever that uke's boundaries have been reached. Some tap right when they reach it, some tap on a hard limit. Still, it's a safety measure ... for training. Safety measures are there for a reason.
And this part:
"Releasing the pin simply because someone starts tapping could become a dangerous habit, presumably because a clever assailant would use that to trick you."
Wow, just wow. How many times have we read threads about the viability of aikido in a "real" fight? Or aikido versus another martial art? And how many times have we read people replying that the chances of fighting another martial artists were slim to none? So, who, exactly, is it that we're going to fight and who will know to use tapping as a way to trick you? And how many times will that come up?
Unless things were completely different than what they sound like upon reading this, I'd steer clear of that instructor.
Consider the number of people walking around in "TAPOUT" tshirts at this very moment. Folks know to tap to get someone to stop hurting them. A few of the bad guys I've bounced have tried to tap out of immobilizations, but I tend to work a lot of cage fights. My best response is to let up a little bit on the pin, but keep it on enough to maintain control until the police arrive. If they keep trying to tap when I know the pin is slack enough to not injure, I will communicate with them, ask them what hurts, and how I can make them more comfortable, like maybe just switch to the cuffs or choke them out if they really want to be unpleasant about things....
Sounds like that instructor was trying to use a very valid point to cover a lapse in skill and judgement. Demonstrating in front of a class is very different than controlling an aggressor. It is a good idea to really practice awareness and zanshin while pinning someone out. It is a good idea to develop as much sensitivity to your uke's position, tensions, and attitude as possible, so that you can control an aggressor safely for both people.