This is potentially a really good idea. It just needs to be worked out a little more.
Some people like to play balanced str/dex fighters, and those of us who do get the short end of the stick on attack roll bonuses. That's a fact.
But I have a beef with this appearing as just a feat that anybody can take during a level up- this idea for balance could be too easily exploited.
Example.
You start your game with a melee fighter with 14 str and 14 dex, then you go on to raise nothing but strength for the rest of the game. Along the way, you take this feat because you've noticed that it will provide you with an additional +2 to attack, essentially being a more powerful weapon focus.
That's no good.
If it were made so that this feat nullified itself if the character surpassed strength and dex balance in one attribute by any more than two points/one attribute score (i.e. strength 17, dex 14 would mean you no longer gain any bonuses from this feat) then I could see this as being a benefit offered maybe by a new prestige class that promoted a balanced str/dex melee fighter.
But, even then, I still definately would not offer the feat as something just anybody could take during a level up.