Anyway, the Federation wouldn't pose much of a threat to the Borg if the virus could be easily contained by the Collective, would it? And once again, PICARD WAS SURE THE VIRUS WOULD WORK!
Again the Borg can cut off infected cubes, so Picard can be sure as he wants to be, but being sure isn't worth shit when it's proven that you're WRONG.
If you were sure the virus would work, then you are ethically obligated to use it, regardless if it actually works in the end or not. The fact that no one said or even implied the virus might not work should have left Picard with no doubt about the Virus' effectiveness. When Picard tried to justify his act of cowardice to Admiral Nechaev, not once did he say he doubted the virus would have worked. PICARD SHOULD HAVE USED THE VIRUS!
Also, I think a Federation of billions is more of a threat to the Borg than a planet of 100,000 people.