Okay, this is just silly... The Smoker, Spitter and Boomer all cause detrimental effects to the area around them when they die. As such, players will generally be cautious enough to push them back out of range and then shoot them in safety.
But the 'kill' command negates all that. Simply drop down or run into the group and hit the bound key, and you can trigger the effects at the exact moment and place that YOU want to. This is astoundingly unfair for the survivors, particularly with the boomer.
That boomer blast has got quite a large radius. Just waddle somewhat close to everyone and then splatter, and you'll be able to knock them back (thus preventing them from shooting for a second or so, and also scattering them slightly) while sliming the whole lot. That is a devastating tactic, and the only way to defend against it is to just not let them sneak up on you, which isn't always as easy as it sounds.
And the Smoker can be just as deadly if used correctly. The smoke cloud he puts off is nearly impossible to see through and prevents characters from automatically warning others of incoming Chargers, Jockeys, TANKS, etc... And there's not a whole heck of a lot you can do about it.
The Spitter is too slow and too fragile to utilize this tactic as effectively, and the effect is more of an annoyance than a danger. She's better off hiding out and waiting for her next big acidic loogie to come along. Still though, under the right circumstances, she might as well die in the middle of the survivors than off in some corner or something.
It struck me a while back just how deadly a coordinated special infected attack could be. Boomer pukes/blasts at least one of the survivors, Charger runs through them and takes one with him, Smoker nabs one of the remaining ones and then a Jockey hops on the second-to-last survivor.
This leaves 3/4 survivors getting helplessly pounded, and the only guy who can save them is slimed. And besides, which one does he go after first? Better decide fast.