Here are my thoughts on pressure points in regards to martial arts...
Ok, pretty much anything that can be measured (in regards to physical aspects) can be measured. Applying that to pain tolerance, you are going to have about 70% of the population with a moderate response to pressure points. You are also going to have about another 15% of the population that has a very high response (low pain tolerance) to the pressure points, and then on the other end of the spectrum about 15% that will have little or no response (high pain tolerance). So, as a conservative estimate pressure points will work with about 85% of the people in normal circumstances. That figure will be slightly skewed due to adrenaline, drugs/alchohol.
So playing the odds, if you can hit a pressure point, it will likely have the effect that you wanted it to. On the other hand, if you know how to hit properly, you are still causing damage to the body whether or not the person felt it, and are still effecting them structurally.
I also think it is important to differentiate the different types of "pressure points". Are you talking pain points, nerve points, bone points, organ shots, etc. In my work, I have hit people with nerve shots and effected the limb to stop working, but they didn't really feel it at all, so I think it is important to figure out what is meant. Now, if on the other hand you are talking about only hitting a precise sequence of small accupoints, then I'm not a big fan of that. I still remember seeing a certain well known PP instructor in a magazine showing him pushing a pressure point in his partner's foot with his big toe, while he activated another one on the attacker's arm while the other hand was activated a 3rd point on the attacker. Huh? Even with a cooperative person he looked bound up and off balance. I prefer to rely on specific medical points, and know "why" it does what it does. I don't have time to check my watch and see that it is the hour of the ram so the small intestine meridian is vulnerable.