OK, let's not argue about what constitutes low or moderate effort, these are subjective terms anyway. Most of the coaching stuff I have read recommends tempo sessions at between 75% and 90% of threshold. 85% puts me squarely in the middle of HR zone 3 and that's pretty much where one derives the biggest aerobic bang for one's buck, as far as I know.
Anyway, we seem to be agreeing that just below LTHR is an effective place to train. I'd still contend that spending too much time above LTHR is counterproductive. Necessary, but if too prolonged it takes too long to recover from that session and probably compromises the training one can do in the succeeding days.