I have seen so many good teachers - excellent subject specialists-chased away by classes like the one described above I'm not nit-picking here I promise. But, I find your expression very interesting. I strongly believe excellent subject specialism in itself does not make a good teacher. Moreover, I don't think that it's the job of SLT to bridge any missing gaps. For the record, I've been teaching for nearly 20 years, never thought about joining the SLT crowd even once. The thought repells me. My parents both ex-teachers , firmly believed that it was the least competent in the classroom who were the first to be promoted! What do I know anyway - I'm currently being managed out on 'capability' if my current SLT have thier way.