WoWDB

First implemented in patch 4.3 after the incident involving  Eye of Purification from  Firelands in which the said heroic item was BiS for enhancement shamans despite being an intellect weapon. It basically involved hard-casting  Lava Burst (which has since become Ele & Resto only).

Explanation of 4.3 Balance Changes, Part One

We changed Mental Quickness and Flametongue Weapon to hopefully put a nail in the coffin of Enhancement shaman using spellpower weapons. We want Enhancement using melee weapons. Players who have been doing so should notice no change in their DPS. Players who have been using caster weapons or armor will find that gear is no longer useful to them.

Patch 6.0.2 note:

Removed. Enhancement shaman mana regeneration has been increased by 100% to compensate.