AP did a super job of contributing to this discussion.
I might refocus on one key tenet: Tempo running, at least as I see it, bridges the gap between necessary easy distance running, which builds endurance, and aerobic power running training, which is, more or less, 5k pace to about 2k race-pace.
I beileve the bridge between the two ends of the "effective aerobic continuum," which is from ~ 60% to 100% of VO2 max/peak/power, is absolutely necessary; if one seeks ultimate improvement in runnning performance. Some authors, such as Owens or Seiler claim that world class runners don't run in-between the extremes of easy and hard aerobic running (so 60-70% of VO2 peak and 95-105% of VO2 peak), but I think they are cherry-picking to support their philosophy/idea. I've talked to, been around, and read about many elite runners who, in fact, train often in the marathon to half-marathon pace range; often ad-lib, and sometimes scheduled.
I recall, as if yesterday, talking to Paul Cummings, who said that "anaerobic threshold" running is the bedrock of his training, and the training of many world class runners he knew. I asked him how fast anaerobic threshold running was to him, and he said, marathon pace." I asked was that exact and he said no. I asked him how often and he said 3 times per week. I asked him how far, and he said, "It varies." I asked what the minimum amount and frequency he used during his best racing years and he said, "5 miles, three times per week. I knew I had to do at least that."
I have go take care of my son, so I'll close now. But, just note this, based on my personal experience, as a runner for 30 years, and as a coach for 20 years: tempo runninng is a form of stamina training, which I believe is vital to a well-balanced training program that focuses on performance enhancement.