sometimes jamming is good; sometimes, not. depends on situation and the person you work with. depends on goal. sometimes i want to test my ability, i would ask my partner to do his/her/its best to jam me. sometimes i would ask my partner to light up because i want to work on my aiki so as not to trigger a lot of my muscle tension. sometimes i would reach out with my fist or foot and lightly tap my partner to let him/her/it know that he/she/it is not in control.
then there are jams that are just hard to deal with (especially the raspberry type) http://www.youtube.com/watch?v=HXKOsajNZY4