If you do not cap ANY resources, waiting for an auto attack and Obliterating on your auto attack has a higher chance to line up a KM proc with that Obliterate as opposed to not waiting.
Here's the cost as opposed to not waiting.
1. There is no cost. You don't cap any resources.
Here are the benefits.
1. By waiting for your auto attack to land, you have a chance of lining that Obliterate up with a Killing Machine proc.
2. Therefore, the damage is increased by (KM proc chance - raid buffed crit chance)x(crit modifier)
If you were gonna crit anyways, so what, nothing gained nothing lost. However, if KM does proc on that auto attack and you aren't capping any resources, you GAIN DPS.
How can people not understand this? YOU DO NOT LOSE ANYTHING BY WAITING.
P.S. I am not advocating you wait for Killing Machine procs. I am advocating you wait for your swing to land.