I do hope you realize the technological nightmare involved in coding an ability to proc on someone else's random proc. It also doesn't fit with the theme of the action, which is essentially using your shield to counter when it's been hit.
What Paladins are lacking is the ability to truly break ahead and do burst AoE. What you're suggesting is a small sustained AoE boost, with 50 potency/9sec max uptime per enemy. When you compare that to the potency a SMN, BLM, or MNK will be able to put out during that time, there's no way it would make up the difference. Paladins would still have to tab target and flash often to keep hate in AoE heavy groups.