I don't have the equations but I remember looking through the impact of defense and remember the additional defense coming out to like, 15 AC :| I still did it though. In terms of cost/benefit it's pretty much near the bottom. I'd only recommend doing it if you like murdering a hundred thousand alarans or enjoy repeating the quest for the fragment a few hundred or whatever times.
A big reason to do it, is because people that have not done it are jealous of you for having earned it. My small clear flowing stone tear speaks from experience.
Be cool if they removed the (10 max) part of the 5% equation. Wouldn’t be game-breaking if Tucoh’s math is correct. But would actually feel like more of a significant reward for a significant grind.
Removing the 10 max would be a nice incentive for it. It wouldn't add much to it but it would make it a little more worth while.