Good or no good?

[Miggiwoo]

It's sadly not that simple, as this would likely make Fending accessories highly desired for DRG and MNK in progression environments, and NIN would have no equivalent option for DEX accessories. The better option is probably to change damage formulas for tanks such that both STR and VIT are included in the calculations to some degree, at some ratio that encourages Fending over Slaying accessories.

That sounds like a good option, you could also make it so fending accessories can only be used by tank classes, or remove some secondary stats.