It seems to me that the same principle works in the reverse, to the PCs favor, as well. The Tarrasque's Will defense is *ELEVEN* points lower than its AC, for example.
Though not every PC can take full advantage of it. Fighters can't target will, for instance. And solos tend to have enough hp to weather an all-out assault from the party, though the reserve is not always true.
It is not all that different from the "target the foe's weak save" concept in 3e.
