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'.
Fair enough, perhaps when I have more experience I will agree completely. For now this is my experience. When an Uke is willing to introduce me to technique I learn faster. Hopefully this won't lead me down the wrong path and set bad habits.
Since I 'm not that far from no Kyu and even now haven't seen that many techniques. I still remember watching Sensei introduce a technique. Thinking the hands go this way, the feet this way the hips that way and he's saying to focus on energy that way; damn how did the hands go, never mind time to give it a go. Getting as far as ich and thinking ni, right ni well that's two now were am I going. He went left and...damn what was it. Getting jammed at this stage of my training didn't seem to help. It left me not only frustrated but befuddled. Being told it's like ten kan remember, did help. Being jammed and not knowing why did not. The best sempai (from my beginners perspective) were the ones that didn't give me the technique but didn't leave me stranded.
I did encounter a few of those that left me wondering how to do the technique even though I "threw" them. I could tell I had flubbed the technique. These Uke left me just as frustrated and befuddled. (Do they have a label?) I could see that these Uke may be more "dangerous" in that if I was not aware could feel I had been a success. I threw them after all. But to be honest, I never felt a success.
Now I don't have your experience, but, when I did my first multiple attacker randori. I did get "jammed" or to be honest it was not a jam, more of two of three attackers had gotten me into a position that I couldn't easily escape from. It took me what felt like minutes but I think it was more microseconds, but I remembered what the sempai that had jammed me and remembered how he had me find my way through the jam and escaped. Next, I hope not to have to remember but to just do but for now at least I did something. If I hadn't been coached I'm not sure I could have found the way out of that pickle. But I had been given a way to think about what to do when I was stuck. That training helped.
I don't think that jamming is inappropriate. I can see where you are coming from. However, as a beginner, my experience leads me to believe that jamming a beginner who hasn't even gotten the basic form to be counter productive. Give me a chance to at least understand if I'm going left not right. Now don't let me go right and don't just give me the technique but don't leave me going left, being jammed, going left being jammed and not knowing if the reason I'm being unsuccessful is because I need to spiral my arm, move from my center or the hundred and one errors I've made to date and the million others I can imagine.
I don't want comfortable training. I don't mind being frustrated if I can learn something. I feel I'm getting closer to being able to learn from being silently jammed with no place to go. But for now leaving me jammed on basic techniques with no place to go and no background to fall back on just what I think I saw from Sensei and what I might of felt after taking a Ukemi from a student only a few ranks above me (didn't he get his footwork wrong? I'm not sure. I could of sworn it wasn't right) and in that process spending much of the time trying to remember the basic Ukemi (do I tuck the inside or outside leg? Bugger it. It's time to fall. Ouch, wrong choice). To jam me can be counterproductive if all it is is a jam. You know you did something wrong but what?! Take the next fall, see if you can learn.