Considering that the usefulness of Defense is so subjective with the D-Level factoring (and it's already possible to beat things like Ifrit Naked on WHM) lack of HP is about the only thing that would keep FFXIV's MNK from turning into a overpowering DD/Tank MNK like in FFXI. I'm sure Yoshi doesn't want that happening so in a way lack of HP is in a way a necessary evil.