Since Breath spells are modded exclusively by HP, I don't know what benefit, if any, you could possibly derive from this shield.
If you're using a Breath build, you probably don't want to get in close to feed the enemy TP and/or get in range of its TP moves. Each HP you lose reduces the potency of each of your breaths, and /MNK means the best you can do is Zephyr Mantle in terms of shadows.
Which would you prefer: +2 INT to play around with Expiacion? Or +10-15% Accuracy/Potency and an additional Accuracy or Potency Staff Grip?
@Eremes: Always had thought INT affected resist rates when it came to breaths, but not so sure anymore. Besides, with the new Harpy Shield, it's outclassed either way. Hofud/Harpy shield is a pretty good combo when doing hybrid physical/nuking.
Also, for breaths, you only get accuracy bonus from staves, you *do not get potency!*. If you're not likely to resist, Hofud would provide more damage and you're still getting some m.acc from Harpy. If you don't have Hofud, then yeah Fire Staff/Axe Grip.