Ruth has a great point about the grip concept. From my little experience a vice like grip may involve a lot of arm muscle use, which is generally not encouraged where I train as it can start one on the path to "muscling" a technique.
As far as the pain idea goes I agree with Peter (of course
). The way I was taught is that there are pins/immobilisations (osae waza) and locks/submission techniques (kime waza???) the only difference being the degree to which one carried the joint in the particular technique - enough to control = pin, enough to cause pain and damage = lock/submission. Both have their purpose and place in training and proper kuzushi or taking of uke's centre as some put it, should actually be an integral part of every
effective Aikido technique imo. Kuzushi is not an option, it is necessary.
Just my few cents.