The best argument against doing that, that I can see, would be because the dev would have to rebalance the effects of enchantments if players could scale enchantments like that per individual enchantment.
Take the Scroll of Charming for example : +10% better prices in your favor and some relationship score boost. Everyone starts at 50% item value when selling. So, if you enchant every equipment slot with the Charming effect, you can neutralize that 50% penalty, and it would be a flat 100% item value to sell. Seems pretty balanced to sell back an item for its normal value.
If you made such an enchantment any stronger or add another item slot. Making sell prices 110%, let's say. You'd easily break the games' currency by buying everything Aldric owns at <100% item value and selling it back at 110% item value. You'd be able to infinitely generate gold until Aldric has no more gold on him.
Similarly, with the Parry Chance enchantment (forgot the scroll name). If you enchant every slot with it and use a staff you can get around 75% or 85% Parry Chance. Which is pretty high already, but there is enough room for you to still get hit sometimes. Anymore, and you can almost guarantee you'd never get hit if you reach 100% Parry chance.
That's just the two things I am aware of when I mentally considered trying to break the game by stacking enchantment when I was playing casually.