As a beginner, I have encountered two types of Jammers. One has been useful in my learning process and the other has left me frustrated and IMHO wasted my time on the mat.
The first jams my technique. Letting me know I have failed. They then help me through the technique either by "leading ukemi" or verbal ques. This method is effective for me.
The second jams my technique and jams my technique and jams my technique and... Then we move on to the next technique or if I'm lucky Sensei comes over to point me in the right direction. In the end all I've learned is that I wasn't doing the technique correctly. I don't know why. I've spent several minutes of bang my head against a wall when I know there's a door around here somewhere.
As a beginner, I would ask that jammers consider where the person they jam is in their learning. If they are just learning what the technique is, don't just jam but lead them through or at the very least encourage them to find the solution (this has worked for me as well.)
I could see how when I'm not just trying to figure out what the technique is that getting jammed would be a great learning experience and even fun as two partners search for "weakness". But for now, when you jam and leave a beginner standing with no place to go, it is just frustrating. When the process moves beyond just what the technique is to how it works and on to perfecting, I can see jamming as being quite effective as a learning tool help or not.
[For me this "problem" has been rare, once or twice. And in most cases I believe from students that could find the mistake and jam but weren't sure how to guide me through a solution. Which in the end is probably fair enough.]
I believe one is coming to the dojo to be frustrated
and NOT to be comfortable. Comfortable training is useless. With comfortable training, students become lazy and don't develop a basic habit of stealing techniques from instructor (by variety of ways). Instead, they are only waiting passively for help like a sheep.
Later on, when time comes to develop the applications facing sophisticated attacks, they have not skills ready to find instant solution. And as in aikido you don't have second chance, from martial point of view, the result is a ‘death'.