What if someone who privately knows about the vulnerability gets the idea to exploit various installations of competitors (or even common users!) during the embargo period? Do you trust large enterprises not to misuse their knowledge to their own advantage?
Of course that's a risk. But again, is it worse to have a handful of people who are trying to be secretive know about the vulnerability while vendors update and carefully test their software, or for for the entire world to know about the vulnerability while vendors scramble to get something out the door as soon as possible?