I am with you on this one. I wouldn't even time the slow days. The great thing is XC is a bit of a meritocracy ... if you aren't on the cusp of like 7th or 8th on the team, it will be hard for him not to run you on varsity if you can beat other girls.
A good coach should welcome the discussion. I feel the biggest mistake coaches make is they don't explain "why" certain workouts and paces are being done. There should be a reason for everything and kids respond better when they know the reason. There is plenty of evidence that taking it easy on the easy days is a good move.
If you do it your way, if you don't have a good race it will probably get blamed on your slow days whether that is accurate or not.
But, give him the benefit of the doubt and see if he will explain why he wants you to do this. And, maybe try it his way for a few weeks and see how you respond.