The M134 is commonly used on aircraft/helicopter mounted applications, and as defensive weapons on some Navy ships. I'm no expert, but I get the impression that "truck mounted" ones like in the film are gimmicks.

I read somewhere that while gunfire in the military is, of course, intended to hit the bad guys, it also has the function of making the bad guys try to burrow into the dirt and think about other things besides shooting guns at you. A machine gun does this quite well. In the movies, it's traditional to show the hero running across a room or something while about ten guys with machines guns pour withering fire at him...and miss. Somehow I think in real life it's more effective.

Unrelated aside: I used a truck-mounted M134 in my fantasy book to dispatch a monster, proving the thesis that guns do kill monsters if you get one that's big enough.