Mihaly Igloi pioneered that method, actually. It's rather interesting; the whole "genesis" of the interval workout is outlined in Kenny Moore's "Bowerman and the Men of Oregon" book. Here's the sparknotes:
*Mihaly Igloi makes his runners do intervals twice a day, almost every day of the week. Very little to no "recovery" runs.
*His runners get very good, but have a tendency to burn out and get "stale" very quickly. Their legs suddenly aren't snappy, they can't run their times, etc. Classic burnout/overtraining from chronically low blood pH levels.
*Another coach (can't remember who!) noticed this and carefully monitored his runners. They still did intervals every day, but the instant one of them looked a bit stale, he would be prescribed several days of easy jogging.
*Bowerman picked up on this, then had the brilliant idea of separating EVERY workout with a day of easy jogging. That way, you get the benefits of hard workouts, but you can still recover from them. Thus, Hard/Easy was born
*Bowerman would tweak this sometimes: some guys, like Pre, could handle two hard workouts in a row before they needed a recovery day. This was very rare, though. Some guys, like Kenny Moore, needed TWO recovery days between workouts. Bowerman's skill was partly in personalizing training for each runner. The U of O's easy days between workouts were, from what I hear, a lark! They'd do 3-4miles easy in the morning, and 3-4 miles easy in the afternoon. On the weekends, some would do a long run. My memory is fading here; track history aficionados, correct me if I'm wrong!