While this feat may not seem broken, on the surface, it does alter game balance among classes and builds. If you look at the PH classes, for instance, there are a number that tend towards high STR (primary, or relatively high-priority secondary) - Fighters, Warlords, avenging Paladins, battle Clerics, brutal Rogues - those classes had better melee basic attacks than other classes that had less (or no) reason to invest much in STR - like Warlocks, Wizards, artful Rogues, and devoted Clerics. Now, either lacking a viable melee basic attack was taken into account when those classes and builds were balanced, and the feat breaks that balance, or the classes were imbalanced to begin with, and were just now fixed. Considering the power selection of the avenging Paladin (there's a dearth of STR powers for it to choose), and the relative merits of some brutal vs artful Rogue powers (sly flourish vs riposte strike, for instance), I'd have guessed that having or lacking an MBA /was/ considered when classes - and, especially, builds - were balanced with eachother.
Warlocks should be particularly pleased, though: they can finally stab people effectively with those pact blades they've been carrying around.