If everything is leveled to your character in the game, it eliminates the challenge. That's probably the most important thing BGS should have learned from Oblivion.
Unique items should not be leveled. Not for the powerlevelers' sake, for the sense of accomplishement.
but leveled items does not mean there will be no challenge. Not every leveled item or creature has to be the same level of you, they can be below or above, so with level scaling you could have an extremely challenging area with an extraordinary reward at any time, but without you would only have a challenging area at a certain level and a good item at a certain level, or you would get the best item in the entire game and lose the incentive of exploring not to mention having static areas would limit your character to certain areas becuase you would get your ass kicked everywhere else
static items/creatures should not exist in an ES game becuase of the scale of the game, they would limit the player or be a huge exploit for them
I am not saying that the level scaling is perfect in the games but it can work if it is used better, I also think making items like deadric artifacts level up with you is a good idea, becuase you cannot get them more than once