A concept I've thought about often and one that Tinman has discussed is the idea of the benefit from a workout likely having a logarithmic progression, where a lot of the benefit probably comes by the 80% point (i.e. 8*1K at 10km pace is going to give you most of the benefit that doing 10*1k at 10km would have).
In regard to what we have been discussing, take for example 10*1K w/ 200m recovery. Doing that at 10K pace is going to be a 100% effort, but doing it at say 90-95% 10K pace is not only doable, but repeatable after 1-2 easy days. Repeating that week after week, does seem to build a superior level of fitness, in my experience, than doing those killer workouts.
I've wanted to try this with straight tempos. Run 10km at 90-92% race pace 3 times per week. Mainly to see what the result would be relative to doing the tempo intervals. I live in Florida though, so the weather isn't ideal for continuous tempos.