For me, it is a crippling glitch. I made my sound the Missile Lock from MechWarrior 2 Mercenaries so it is much more annoying and demands your full attention for a good reason, but when it doesn't play, I cannot tell if I have incoming.
Uhh, yeah? You don't want to slide your butt into the thing, so you listen for the beeping and see if it is getting faster or slower in the direction you are going. There are a lot of things that you can do with a simple audio and visual cue flashing on your screen.
Funnily enough, I seem more than capable of playing the game just fine regardless of whether or not I have the missile lock functioning, hence why I never really notice when it's there or not.