Let's simply start with the Ikkyo pin. You are face down on the mat. The nage has graciously not driven one knee on top of a kidney, or into your floating ribs. The other knee was not driven down on top of your neck, or on top of your elbow. There are a host of other options which would preclude the person from functioning well (if at all) from that point forward. The nage is still free to move, either releasing the person on the ground, or causing pain and injury to the uke who does not truly recognize the dangers of being placed in that position.
Pins done very well can immobilize a person. Even if the person is not fully immobilized, the idea that you are going to fight your way to freedom while being so exposed to injury is simply comical in a Darwinian sense. Pins are simply pauses in time. Imagine waiting 1/2 hour in the Ikkyo pin in the middle of a bar, waiting for the police to arrive. Just as comical as thinking that the pin is an end-all to a situation.
One of my new students has me by about 75lbs and at least 5" in height. Good boxer to boot! I had him in an Ikkyo pin and he decided to test the pin. Every time he moved, I let some part of him know that he was being struck. He decided to move onto his stomach and up (like in wrestling). A simple rear-naked choke finally got across to him the futility of what he was doing.
To me, if you are in a pin and exposed to being genuinely hurt and you insist on thrashing about, Uncle Darwin is smiling down upon you..... The pin is a pause for both sides to take note of the current situation and respond accordingly. The graciousness of the pause should not be mistaken for weakness or strength.
That is the thing in "aikido" pins that is overlooked. The pin in and of itself may not be the BEST way to hold someone down, but it is the other things that go along with it that allow you to keep them down. There is a balance between your level and investment in a pin and the risk...for most of us that is.