If you need it and have a budget for it, then it's foolish not to have the equipment.
My instincts tell me Arpaio probably got this baby more for show then to blast away bad guys. Still, if having some heavy duty firepower on hand deters the bad guys, then the weapon has paid for itself and you don't even have to fire it.
On the other hand, U.S. sheriff's deputies and Federal Border agents have been confronted by Mexican Federales armed with mounted .50 caliber machine guns on U.S. soil while our guys only had their side arms. - Not a very fair fight, if you ask me.
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Hebrews 13:23 Know ye that our brother Timothy is set at liberty