If a teacher can't explain ki, then he's not worthy of teaching aiKIdo.
Hmmm ... lots of excellent teachers believe in teaching silently, or nearly so. They don't explain anything. They just demonstrate techniques, and their students learn by watching. This builds up an amazing awareness and sensitivity. (Other excellent teachers teach using other approaches, and some talk all the time, of course.) Is it "can't" or "won't" or "doesn't" that you mean? Also, how do you judge an explanation of ki?
The point I was trying to get at earlier, though, is that ideas about 'ki,' ideas about 'ai' (like in the tenkan example, or teachers who emphasize the effectiveness of soft technique) and ideas about spirituality and the great oneness of the universe tend to get mixed up in our discussions.
While all of these connect to the 'spiritual side of Aikido,' I'm not sure that they are all exactly the same thing. I can easily call to mind teachers who emphasize one of them more than the others. However, I can't really call to mind any who emphasize the third, 'spiritual' aspect.