I hated when some one would use kb then say deal with it and chose another target and just to have that target kb'ed.
See, this I can understand, because KBing one target then switching to another target and KBing the new target before the old target is dead is just stupid... Not to mention rude to the Melee.
Of course, if I had a toon with powers that had even a chance to KB a target, then I'd focus on one target so that I don't make problems for the Melee by making all the targets fly away, Granted, that's rather hard to do if there's only one baddie left, but at that point, that's okay because it's just one baddie versus multiple baddies
(On a side note, I have a really bad urge to start quoting Sergeant Major Avery Johnson, aka Sergeant Johnson, and his speech from the beginning of Halo Combat Evolved that differs in what is said depending on what difficulty the Player is on... Legendary has the best speech
)