Pup of Cho'gall asks a very sensible question on the WoW General Forums: since the enchants for one-handed and two-handed weapons are often different, and since you can enchant shields, why can't casters enchant their offhands?
Tseric responds by saying that offhands are a pretty broad category, and that there would have to be a lot of different enchants to give casters all the options they might want in that slot. That seems like a pretty weak excuse to me, and most of the forum posters seem to agree, noting that they'd be happy to get some of the enchants available for shields on their offhands. Stamina and intellect are particularly useful to caster classes, and it wouldn't be too hard to make a +damage enchant for shields or offhands.
Someone else suggests later in the thread that offhands aren't enchantable because their ilevel budget doesn't factor in enchants, which would make a lot more sense than the official explanation. I'd compare the stats between an ilevel appropriate 2-hander and a one-hander plus offhand, but I'm sadly not a caster and would probably muck it up. It's also true that most of the decent end-game caster enchants just specify a "melee weapon," not a one-hander or two-hander. Still, it's an interesting debate.
Several casters I know use mostly two-handed weapons just because they're sick of getting an offhand or a one-handed weapon, waiting for an appropriate item to use in the other slot, and then having limited enchantments. Admittedly, offhands have the postive of usually looking really cool, but they're still kind of a pain compared to the big clunky two-handers.
Do you think shield enchants should be expanded to include offhands? What kind of enchants would you like to see on offhand items?