I don't think its cheesy (actually fits a character concept I had with a hexblade in 3.5). It may not be what the designers intended by RAW, I agree.
Anyway, here's the related text from the compendium.
Compendium: Hexblade said:
You can make weapon attacks with your pact weapon, using its proficiency bonus and the appropriate damage die. Your pact weapon shares your implement’s enhancement bonus, critical hit effect, properties, and powers. The weapon cannot be enchanted.
When you use a power associated with your pact weapon and the power has both the weapon and the implement keyword, you are considered to be wielding both your pact weapon and your implement for the purpose of feats and other game elements.
So:
- Properties, Powers, and Crit Effects transfer. Thus, a Flaming Drpw Long Knife transfers the Flaming power to the Hexblade Weapon/Implement powers
- Groups don't transfer (so no Light Blade Expertise on the Heavy Blade Hexblades)
- If I have Arcane Implement Prof (Heavy Blades) and wield said Flaming Drow Long Knife in the Off hand, I am wielding two implements. Dual Implement Spellcaster applies*.
- I am also wielding Two Weapons; Two Weapon Fighting applies.
* TWF always applies. The only nit is in the whether the last sentence in DIS precludes the use for Hexblade powers:
Compendium: DIS said:
When you use an arcane attack power and you are wielding a magic implement in each hand, you can add the off-hand implement’s enhancement bonus to damage rolls.
Both of your implements must be usable with this power, and you must be capable of wielding both implements, to gain this benefit.
Are both usable with the power? The weapon is usable (and is in fact
required to be used). This nit comes when you realize you cannot use the power with just the weapon; you have to have an implement in hand to summon the blade in the first place:
Compendium: Hexblade said:
While you hold your implement in one hand, you can use a minor action to manifest your pact weapon in the other hand.
So, the power is not usable with the Blade by itself, but it is usable with the blade and the implement (and in fact, must be used that way). The feat does not say each has to be used individually.
I am comfortable in saying you can use DIS with a Hexblade. It doesn't beat out the damage of a Slayer, say (not at Heroic, anyway). In any case, you can still do the "Make my Hexblade Flaming/Frost/Lighting/whatever" trick w/o DIS.