The main reason they are commingled is that teachers need to have mastery of what they teach. As subjects become more difficult, fewer and fewer people have the required mastery to teach a subject. At the end of this (i.e. graduate level courses) the only ones with sufficient mastery are those that are doing the research.
For lower level subjects (i.e. most undergraduate courses) this is much less an issue, but the above still explains why we need researchers to also teach. Issues occur when you let a brilliant but eccentric mathematician teach calculus 101.
His research is in no way relevant to the course, and he might not be able to explain things on that base a level.
> At the end of this (i.e. graduate level courses) the only ones with sufficient mastery are those that are doing the research.
Sadly, sufficient mastery of the subject often comes at the cost of sufficient mastery of teaching, which becomes especially important for such difficult subjects. If only some of that money spent on more staff was spent on assistant teachers whose main skill was teaching and coaching other teachers, and providing paid hours for both of them to fix the curriculum together...
Anyway, I fully agree with your second paragraph. Another issue is that of researchers not really seeming to care, having an attitude of "you shouldn't be in University unless you really want to learn this, and I'm not going to try to motivate you or help you with that." There's some truth to that, but it also sometimes feels like it's used as a way to mask their own teaching incompetence.
I was asked to be an assistant teacher after graduating from my masters, and I completely freaked out about the responsibility of making sure I wouldn't fuck up my student's education (I cared more than most of my students did). I almost declined the job offer! I was basically told "yeah we know, that's part of why we want you."
Anyway, I spent a lot of time on-line trying to find advice on the dos and dont's of education. I can say that the best two lecture were Eric Mazur's Confessions of a Converted Lecturer[0] and John Corrigan's Are We Listening To Our Children?[1]. I think every teacher should watch these at least once.
Honestly, when mastery of the subject becomes detrimental to teaching the subject, your not taking the highest-level courses.
A great researcher has to teach and convince themselves, those skills translate to teaching at that level.
I'm mostly thinking about the highest level math courses though.
I guess you're talking about the point where it's more collaboration on a half-solved problem than data transfer about solidified knowledge? Because then I agree, but that's a really different type of knowledge transfer we're talking about then.
There's a point before collaboration, where you ask a researcher to bring a student up to the level of the researcher. At that point, the student and researcher struggle with similar enough problems that the researcher can think from the students perspective.
For lower level subjects (i.e. most undergraduate courses) this is much less an issue, but the above still explains why we need researchers to also teach. Issues occur when you let a brilliant but eccentric mathematician teach calculus 101. His research is in no way relevant to the course, and he might not be able to explain things on that base a level.