Eratta removed the fact that magic items can only be damaged by equal or higher plus. The eratta starts: "Delete the first sentence..." The first sentence says: "An attacker cannot damage a magic weapon that has an enhancement bonus unless his own weapon has at least as high as the weapon or shield struck."
And why shouldn't armor also gain +10hp and +2 hardness per plus? Because it can't be sundered while worn? So +5 full plate sitting in the corner is no harder to destroy than normal full plate sitting in the corner? Something built to protect from blows better than normal armor can't take the heat any better? Spread the joy, let armor also be improved with magic.
Ciao
Dave