I can see the argument for the "anywhere in the unit removal". Combat is not turn based, and therefore you can't drop a shell on a unit that's standing still. Presumably, they are moving, breathing, thinking beings who are not fixed in one place. Yes, I know 40k isn't like a real battle field, but I said I understood the argument, not that I endorsed it.
In all reality, they do it to prevent commander sniping.
And in a moment which saved the galaxy from complete annihilation, the Ork put down the Pulse Rifle, knowng nothing of it's strength and rapidfire capabilities, but only that it didn't make a loud enough noise and that it tasted like the bad parts of a squig.
Ork Dakka-ta-Death Army- 2-0-0 Because nothing is more insulting then being out shot by BS 2 Orks.