At least for now, it never goes quite THAT far.
AI tends to *hate* moving. It would only do that if you were literally immune to physical attacks.
Generally speaking, you get the best mileage out of defensive abilities when your enemies won't have many easy opportunities to hit other targets. If I made a 53% block character, I'd probably try to craft a party that was largely support/ranged demons so that my 53% blocker was the only one in front. Granted, you still have to protect him from being focused fired by stuff the blocks won't cover, but being able to shrug off a lot of physical attacks is pretty nice (especially since this also shrugs off any bad status riding on them.)