Re: People who are never uke
Occasionally, I have also met such individuals. Most often they are perhaps advanced students -- although moderately ranked -- from about nikyu through about shodan or nidan, and seem oblivious to any implied or real risk of injury, avoidance of which is what so motivates my own approach to ukemi. As a practice, it can be very, very dangerous to resist taking ukemi, and doing so also causes a person to miss out on half the practice. Besides which, for every action taken to oppose a technique, there is an appropriate application in henka waza (changing technique). Have you talked with your sensei about your concerns?
There are two ideas that come to mind for me regarding what they could possibly be thinking. One idea is that, by refraining from engaging in a committed attack and reactive ukemi, it is possible to maintain control for the possibility of kaeshi waza (reversal). The other being that they have some specific thing in mind that they want their practice partner to do, but they either can't or won't express it. The habits of resisting off-balancing attempts and not committing to attacks are also common among the people I've met who played judo, tai chi, or kung fu, then switched to aikido. It's possible that they're egotistical bullies, but it's also possible that they simply think that their way is correct or good in some way. Have you discussed ukemi and the role of cooperative practice in aikido with the people you are posting about?
Incidentally, nearly all of the highly to very-highly ranked individuals (godan through hachidan) I have had the privilege and honor of practicing with have each given ukemi that was completely appropriate to whatever technique we were studying at the time. Some very highly ranked teachers go around to take ukemi for everyone in the class, as a way of both teaching and evaluating at the same time.