3 months is more of a joke than a reasonable time, but one can argue about that if he wants...
> even users of non-proprietary projects
Actually many FOSS projects get only notified on the disclosure date.
Hiding the vulnerability for such a long time makes more harm good. The vulnerability can potentially be exploited by security agencies that necessarily know about them and could also be leaked to a bad actor by an employee of one the vendors.
Hopefully WPA2 isn't that important, but potentially security sensitive users trusted something that was known by some to be vulnerable for 3 months! Bad actors could have used it against them.
The embargo resulted in potentially bad actors knowing about the issue, but not vulnerable users.