Safer software for all, because it's a better thing that VUPEN discover the bug than if it's discovered by some criminals who keep it secret and scam/hack
I suppose it depends on the point of view, but having it exclusively in the hands of governments can easily mean it's limited to criminals who keep it secret and hack.
Why would a entity as big as "the government" would invest in breaking one browser used by a minority (~10%) of users in the web? Wouldn't it be much easier to just compromise their Internet connections?