Off-hand is a pretty broad category. Adding enchantments to such items doesn't seem to lend the same focus as other categories might. Enchantments tend to have certain restrictions by slot. Boot enchants might be speed-oriented, which doesn't make sense for a chest slot. Chest slots might have additional armor which doesn't make sense for gloves. Gloves might have an agility bonus which doesn't make sense for a helmet. And so on.

Off-hand is composed of shields, weapons, and items both frivolous and advantageous. To start tossing out additional stats into such a category could get sloppy easily. A shield enchant isn't going to be worth anything on a caster off-hand, so you have the potential of needing to generate a multitude of enchants for all these little distinctions, while adding little to the bottom-line. In the space it takes five enchants to be useful for different slots, you could very well need just as many enchants for a single slot. This could become bloated and full of clutter. Just something to consider.