I don't see how it is worthless without the fighting style. You will be adding your Charisma modifier with both weapons.
...Starting at level 12. What did you plan to do until then? In most campaigns, you'll spend the large majority of your time in single-digit levels. You may not even get to 12th, and if you do, it'll probably be getting toward the end of the campaign. A build needs to start delivering
long before 12th level.
You only get one pact weapon. That means your off-hand weapon isn't a pact weapon and you don't get automatic proficiency with it. The only finesse weapon warlocks are proficient with is the dagger, so your off-hand attack without the fighter dip is doing a whopping 1d4 damage. I mean, I guess it's better than
not doing 1d4 damage, but it's pretty feeble. Dip fighter, and you go up to 1d6+3 instantly. If you're a human, you can get Dual Wielder as your bonus feat and go up to 1d8+3. And every time you pump your Dex, you're adding damage.
The extra Dexterity damage on your hand would be nice, but I am not sure it is exactly better than progressing your levels in Warlock.
For a melee bladelock, I don't see much down side. You do have to wait a level for Thirsting Blade (since you can't get it until you have the Pact of the Blade, and you can't get
that till warlock 3, and your next invocation after warlock 3 is at warlock 5). After that, though, it's smooth sailing. Put another two levels in fighter along the way, and you can get Lifedrinker on schedule at 12th--plus you get Action Surge and your choice of battlemaster maneuvers or eldritch knight spells.