An AOG at Sikorsky or at Gulfstream gets everybody's attention - everybody. Robbing the line is certainly drastic, but always done if that is the only way to get a customer up and flying. We even rob the company owned aircraft, and AOG them, if necessary.
The only exception is when a military customer insists on his own spares stockage, and does not sign up for (and pay into) the rotable pool of spares that is available. Then they made the choice, and are usually made to stick with it (although even then, the company can't stand to sit by while an aircraft sits.)