I believe verbal communication with lower ranks is important. If you are training with lower grade, in particular newbies, they miss a lot of points. I guess mainly because they almost never do ukemi for sensei so they can't feel it, and secondly they don't know what to look for. So I believe it is important for sempai to help them through. But I must admit I try to keep the talk down to a minimum in order to maximise the reps. Corrections by sempai on the mat is a huge part of our learning. I have found that learning technique from a sempai with a similar body shape to mine is invaluable.
My Sensei said once, I'll give you the gist of it, our aikido is like a diamond. It is originally rough and uncut. Each time Sensei or a sempai teach you something, it's like a diamond receiving a new facet/face. The more facets the diamond gets the more valuable it becomes.
I believe that if you only learn from the one person then your diamond will become like thier diamond, and I believe it is important to create your own.