Sure would come in handy on a barricaded suspect who's equipped himself with a gas mask and lots of ammo.
Rescue ops are another use. Dealing with riotous crowds another.. Maintaining order in times of large scale disasters is another.
If it encourages even one suspect to surrender rather than fight it out, it's worth it.
So what exactly are you going to do with a .50 machine gun when you have a barricaded suspect, bring the building down on top of him? Be serious. There's a well known saying which is that "if you can see them, they can see you." If he's firing on officers they can certainly see where he is and return fire, no need to bring the building down. Assuming, however, that for some reason you cannot take the shot, you have your local SWAT team make entry, use a flash bang or a stinger, and extract him. If deadly force is needed, so be it. If he can be taken alive, even better. Bottom line, you don't need a fully automatic .50 machine gun to solve a barricade situation even if the subject has a pro-mask and a few thousand rounds. And in the very unlikely event that you should actually have to shoot through the wall to hit him, a Barrett .50 can certainly do the job.
As for 'rescue ops' and 'riotous crowds', Police DO NOT utilize suppressive fire or fire bursts from automatic weapons into crowds. If you hit an innocent person under those circumstances you will be charged with murder or aggravated assault and prosecuted accordingly. If you have people using molotov cocktails in a crowd you use well aimed rifle or pistol fire to neutralize them not a burst from a .50 machine gun. Even when I was in Iraq we had strict orders not to fire bursts into a crowd to hit a subset of people. That's what aimed fire from my M-4 was for.
Fully automatic weapons primarily designed to destroy light armored vehicles and aircraft are not needed in civilian law enforcement.