I trained for a while with a sensei that I was truly afraid of. I was moving to a new city, and had the choice to go to several different ones, but chose a particular city so that I could train with this particular sensei because of my fear. I thought that if I was having such a strong reaction, that it would be good for me to go train with him/her.
My fear was not unfounded, as this sensei sometimes did things that to me now seem inappropriate, just to get a fearful reaction from us--challenging us. I never felt like I could trust him/her when taking ukemi for him/her. One time s/he even popped my shoulder out of its socket during a pin. I don't know if it was an accident or not, but it definitely showed me that I should not trust him/her.
I learned a huge amount with this sensei, and owe him/her a lot for what s/he taught me. My ukemi especially improved amazingly!
In a lot of other ways this person was an amazing teacher, one of the best that I have trained with for a period of time.
Eventually the stress got to be too much for me, and I had to just quit aikido altogether. I thought that I would never go back--I got headaches and strange emotional reactions (shaking hands and intense nervousness) just from doing rolls in the park one day. My body just did not want to do aikido anymore. It turned out to be just a long break (6 years) and eventually I came back to aikido in a new city and with a new, more compassionate sensei.
So, my answer to the question is, yes, I learned an amazing amount from a sensei who purposefully used fear on his students on the mat, but I think that s/he would have been an even more amazing teacher if s/he had not done so. At the very least, I might not have had to take such a long break to recover.
ps: sorry for the "s/he stuff", I'm trying to be discrete...