Wow! I am that "problem student!" As for the unconventional ukemi that week, my knee was injured while I was playing soccer, and I couldn't bend my right leg, so I kind of rolled over the outer blade edge of my foot and down to the mat. Now, since the original poster usually puts his considerable weight into accelerating his partner toward the culmination of whatever movement he does, I will make no apologies for taking proactive ukemi — it is simply for my own protection. Likewise, with tapping for pins; I have no desire to have injured shoulders, and I tap at the natural extent of my range of motion. If only I had the time to address each of the speculative issues at odds against me here in my absence! LOL!!!
Trying to protect yourself is crucial in Aikido, that is what ukemi is for. If you take proper ukemi not just falling, that is accepting the energy that nage is projecting you absorb the impact and protect yourself properly. Proactive ukemi, moving before nage moves you is not really helping you to train in "proper ukemi". Yes, you do protect your self but lose the musubi or connection with your partner.
As far as tapping, I dont think anyone in the dojo is trying to rip your shoulder off or there would be a lack of people left in the dojo. Even if you are injured taking a shoulder pin will strengthen your muscles and tendons. Taking this ukemi does help protect you from further injury to your shoulder. By the time you reach black belt your shoulders and wrists should be much stronger and take more pressure. By not taking this ukemi you have a far greater chance to be hurt later if you work with a strong partner, especially when you are a yudansha.
No one wants to be injured at the dojo, this is where trust comes into play. We trust our partners with our bodys to be harmonious with each other.