Not sure if I agree with this. If the purpose of easy fights is just to let the player relax why have them at all? I think a game should strive to make every encounter challenging. If an encounter doesn't challenge the player in any way, its worthless.
I think the level of challenge should change but all encounters should have some challenge. You should be able to die during every fight. If there is no fear of dying, there is no enjoyment.
I have to disagree. Easy fights are necessary to create contrast with boss fights just like you need out of focus background in photography to make the subject stand out. If everything is highly challenging, the boss battles do not stand out as much. There should be some challenge in random encounters, but it shouldn't be a life or death situation.