So far, it seems as though Blizzard has rather randomly assigned caster weapons to be either "Main hand" or "One-hand." I'm not aware of any ilvl implications of the assignment either way. Until now, no caster has been able to dual wield, so it didn't matter.
In the TBC, you could potentially spec 0/31/30 as a Shaman, retaining good healing spec and picking up the ability to Dual Wield. This means that with current gear, you could equip Scepter of the False Prophet and The Widow's Embrace and then enchant both for +458 healing. By comparison, examine various versions of Atiesh:
http://www.wowwiki.com/Atiesh
Obviously, of course, the "upgrade" here is slightly less than what I have shown since Shamans can already equip "held in off-hand" and shields with +healing.
Does anyone else see this as a problem? Should all caster one-hands be changed to 'Main Hand'? While dual wielding caster weapons is a creative way to increase stats, it seems it would scale FAR too well (since you're operating on two ilvl curves instead of one) and the arguable intent of the Enhancement tree is for DPS, not for improving healing ability.
If not, why? Are Nature's Blessing, Imp Chain Heal, and Earth Shield so good that we shouldn't worry about this? Is it obviously the intent of the designers to let dual wielding shamans use twice the healing weapons? Do Shamans need this to compete for other reasons?