I'm not thinking martial strategy for a moment, I'm thinking inadvertant injury. If nage is pretty focused on applying the nikyo and the beginner is relatively inexperienced in taking ukemi, my experience says that a lot more force develops at the wrist before uke really realizes it when uke has his arm locked. I'm not sure exactly why that is: increased lever arm with the extended arm, maybe, or just extension of the arm preventing flexion of the knees. I don't know, but that's my experience.
As far as martial strategy goes, my understanding of how 'fights' work is very limited due to a (blessed) lack of any real experience. What I was told, though, is that a straight arm when your wrist is even partially controlled is an opening for nage to break your elbow.