For most people fine, but you can't take easy days for optimal fitness at an elite level. Nearly every top elite has to do threshold quality runs day in and out to maintain race pace fitness for most of the year, especially these days. There is such a thing as junk miles, and individuals who ascribe to them hardly ever get past mediocrity, except in rare cases.
Examples; Coe ran every day hard, Maree trained at 4:30 mile pace and never ran slower, Bekele and Geb never ran slower than 5:10 pace on their longest runs, Viren never ran slowly even in base phase, Farah never took easy days, Jonesy was out the door on long runs at 5 minute pace, Mantz never runs slow, Radcliffe didn't take easy days, Dibaba and Defar never trained slowly, El G never ran slower than what equates to 4:50 mile pace, all the current young guns run thresholds and never train slowly. You can check literally tons of workout data.
The one major exception was Steve Cram did write in his book that all season long on Sundays he would jog about 10 minute mile pace for as long as 30 miles. He believed this Lydiardesque recovery run developed his aerobic and capillary system. Lydiard's guys themselves hardly ever ran easy days, even on long runs the pace was steady. Only in the first 2 weeks of base phase did they jog 20 miles once per week to safely adjust to the volume. The runs after the first two weeks were either medium or hard. Email any great runner. Ask Ritz or Meb if they ever ran slowly on purpose or if they believe that junk miles have any real benefit. Peace.