What better way to prevent that from happening again than having an independent mind overseeing the collective?
"Q Who" actually had this discussion.
TROI: We're not dealing with an individual mind. They don't have a single leader. It's the collective minds of all of them.
PICARD: That would have definite advantages.
TROI: Yes, A single leader can make mistakes. It's far less likely in the combined whole.
And guess what happened when they introduced the Borg Queen in Star Trek? She made mistakes! She actually trusted Data, she blew up several of her own Borg ships just to destroy one or two free minded drones, and she blew up herself despite being in a BORG WARSHIP in an attempt to prevent Janeway from escaping.
I'll say this now. The Borg Queen WAS A MISTAKE
. It was a beautifully acted and well designed character, but it did more harm than good. The moment you give a formally non-single entity force an individual leader who controls everything, the less intimidating they become because you now have someone you can defeat. You think the Thing from John Carpenter's "The Thing" would have been as effective if there was a leader thing who gloated at our heroes and when killed, everything was fine? the Borg should have remained "The Thing" of Star Trek.
That's a great point, the limitless unity of them is one of the scariest aspects of them and the queen really hurts it.
The notion that killing any one or even a bunch of the individual borg really doesn't matter because they can just make more. Also, does the individual borg actually die? They've established in general science fiction and Star Trek in particular that the mind of an individual can be "downloaded" into another body, the borg conscience may just be in storage until another body is found or just exist as extra knowlege in the matrix (no not that Matrix!), therefore making them nigh immortal. That is scary. Like a disease, wipe out billions but the remaining few germs can kill you, the borg can't be stopped.
Then they get a Queen and kill her and bang they're dead.