Xeviat
Dungeon Mistress, she/her
I had never thought of using the DMG on monster balance to figure out the impact of magic items, but it is a pretty genuine idea to help feel out the level of power to throw at parties. We've recently been experimenting with an XP rule from a different game, where the difficulty of the encounter matters more than the monsters within it. The easier a party overcomes the challenge, regardless of what methods they use, the less XP they receive.
The idea of being able to quantify magic item impacts on effective level, especially temporary ones, would help us immensely in this endeavor - thanks for the inspiration, @Xeviat.
I'm starting on a big project to rewrite the Monster Manual and make it more like 4E, so I'm going to be doing a lot of testing. While I'm at it, I figured I'd might as well take a look at the PC balance. It's all just guidelines.
And yeah, not all magic items are created equal, but it's a starting point, rather than just saying "eh whatever rebalance the encounters yourself" without having a direction in mind.