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.
I cannot reply on behalf of the OP or the person stating he is the problem uke.
I can say that when a person trains with a disability, he may not be able to meet you standard for "proper ukemi" but still is being the best training partner he can.
I can also say from many years of experience that even if you leave out the small group of people who actually are happy to hurt others, that there are plenty of people who unwittingly tweak or rip their partners because they only have one set "on" level for either/both speed or intensity regardless of who their uke is. There have been plenty of people I have not trusted to train with over the years.