One thing that all of the senseis that I have trained with say to me about aikido. In perfecting and training techniques and the practice of aikido, its about control and blending and not about speed of training.
I find training with beginners most beneficial becasue I can train on controlling the technique and the uke to allow them to ukemi safely because, initially, they are not conditioned to ukemi in a predetermined manner.
I can still train to execute the technique properly but I have the added difficulty to control uke and ensure he/she is still off posture and unbalanced but still guide them into the proper ukemi. This is excellent training in my opinion.
training at speed and with experienced aikidoka is easier because there's is momentum and knowledge the uke will ukemi properly, but to slow it down and focus on pure control while ensuring your uke is unbalanced, that's more difficult in my opinion.
IMHO, training with beginners is a very necessary part of improving aikido and I look for every opportunity to train with them.