Just imagine: close quarters combat turns hairy, as your opponent has just taken out a knife. The blade zings past your skin, and you struggle to hold him off. You manage to tangle his hands with one of yours, just enough to draw this weapon. You plunge the blade in deep, feeling him twitch against you.
And as a coup de grace, you fire a .45 slug through his chest, and emerge victorious.
Best weapon in zombie apocalypse is a 9mm or .40 caliber semi-automatic carbine, like a Kel-Tec Sub-2000. Common, light ammunition, combined with medium-close range utility and compactness of system. Bayonets are even more useless in a zombie apocalypse; if the zombie is close enough to stab, you should already be running.