Nobody gets a trophy for winning the Tuesday easy run.
Fast pace on an easy day isn't fast enough to give you much training effect, but it is fast enough to hinder recovery. The next time you do a hard day, you aren't fully recovered and cant go as fast as you could, if fully recovered. If you extend this out over a season or year, the net result is that you run medium pace all the time.
In college, we derisively referred to these types as "easy day racer". Occasionally, there would be a mock Breaking News report at the end of a run, asking the winner how it felt to win the Tuesday easy run.
Hard days, hard, easy days, easy.