- The defensive value doesn't stay the same. I proposed the change to magic armor in tandem with a change to magic weapons; magic armor thus doesn't always provide the maximum protection.
- The defensive value isn't always the equivalent of AC 1. The 3LBBs only have magic plate, but magic chain and magic leather armor could easily show up. Players expect that they exist.
- Plate armor, even magic plate, is not AC 1. I believe in hard armor classes with no overlap; plate armor is AC 3, even if it is magical and has a defensive bonus. This is especially relevant for those using weapon vs. AC rules, since those depend on the real armor class, not the equivalent due to bonuses.
- I'm trying to get away from the numbers. Lately, I've been referring to Light, Medium, and Heavy armor instead of AC 7, AC 5, and AC 3. This is so that those who prefer ascending AC can use their own numbers with stuff I've written up.
So, I'd rather say that magic armor provides a 2-point defensive bonus against mundane weapons, since that covers either ascending or descending AC systems as well as different varieties of magic armor.