Enhancement's affection towards spell power weapons has two main reasons - one, almost 50% of our single target damage and probably over 80% of our AoE damage are affected by spell power, and two, our scaling with weapon DPS is quite low; thus, any reasonably slow spell power weapon comes out grossly overbudgeted (using my own stat weights, the Eye is almost twice more DPS than a comparable Agility MH). That said, the Agi MH/OH route is fairly competitive and quite a bit simpler to play and Blizzard have repeatedly tried to steer Enh away from spell power weapons; thus, I personally haven't used a spell power setup so far.

As a general rule, enhancement wants a melee weapon in their mainhand. This is due to Stormstrike and Windfury both hitting quite hard and being a nice portion of our damage, as well as because auto-attack damage is still a very vital part of the spec. On top of that, melee weapons come with Agility, and Agility is our bread-and-butter stat, for which we can't get enough of.

However, A LOT of enhancement's damage comes from spells. Maelstrom Weapon procs instant lightning bolts, unleash flametongue does spell damage, our shocks scale with spellpower, lightning shield scales with spell power, etc. On top of that, despite the fact that point for point spellpower is way worse than Agility, spellpower weapons have a ton of spellpower on them, to the tune of four or five times the amount of agility on an equivalent iLevel melee weapon. As a result, in a few specific cases, caster weapons theoretically out DPS melee weapons.

Eye of Purification is one such case. Because of its 2.4 speed and not terrible melee damage, we use it in our mainhand imbued with Windfury, while using a regular melee weapon in our offhand with Flametongue. Our spells see a huge benefit from the spellpower, and it has delicious mastery on it...as well as a nice chunk of spirit that turns into spell hit for us. (Though we'd reforge the spirit to hit, so as to get some melee hit out of it as well.) Lava Burst also enters into the enhancement rotation at this point; we fill in global cooldown gaps with Lava Burst. Since the mainhand weapon is lower damage than a normal melee weapon, the damage we'd be losing by stalling our weapon swings with a cast is made up for by Lava Burst damage.

All of that said, it's fairly advanced stuff that requires 4piece tier 12 and an Eye of Purification. Blizzard's attempts to devalue spellpower weapons for enhance have been somewhat successful, and in normal course of play a melee weapon will almost always be the better choice. (The exception to this is AOE, where caster weapons still shine.) 95% of the time, a melee weapon is the right choice.