Because so many Enhance spells scaled so well with spell power, it was an over all gain in DPS to trade weapon DPS for weapon spell power. Recently, they buffed Windfury procs to three hits, making MH weapon damage more important, and changed Flametongue Weapon to only scale off AP (for Enhance shamans). There's also the rumor that Lava Lash, based on some old talent designs, benefits from both AP and spell power. This might even have been stealth-fixed recently.
In any case, the benefits of using a caster main-hand weapon have been nerfed, and will be nerfed if it ever pulls ahead of using two melee weapons. The safest thing to do is to always get the two slowest, hard-hitting 1H Agi weapons that you can find.
Dread Lörd Kaolian wrote:
As a forum admin, you... see things. Things that you can never unsee. Things that make you fervently wish to protect other people from ever seeing those things. This is why forum admins are sometimes cranky.