A BM hunter's kill command casts per fight represent at max about 5% of the total hits on target. So having that "rare" miss (you can still be very unlucky, just like you can be very unlucky and not crit as often as you'd like) being that specific KC BW Crit is pretty damn small, to a point that it is negligeable. It keeps getting unimportant the closer you are from hit cap, in which case, there is a TON of other factors that would affect your final damage output in a fight that that very rare critical miss.
So let's say you are really that unlucky, it happens, and you miss 3 times during a 6 minutes fight (with a 7.40%, the odds are about 1:1000, and each of these attacks were actually these BW KC that would have crit, so let's say, 400k (like you said, I personally don't get that) 3 times, it does 1.2 millions!!!!..... in a overall 100 millions damage over 6 mins.
I've been very generous with my numbers, they are inflated to make you point look good. Still..
You would have a chance in a thousand to miss a bit over 1% of your overall damage done to a 25 Heroic Boss. 1%, 1 in a thousand!!! In reality, where if you miss, the odds would probably be a auto-shot miss or a un-crit arcane shot, then you dps loss would be under 0.3%