Only by casual hackers. The pros will probably have been exploiting the flaw for weeks or months against gainful targets.
If there is a reasonable end-user workaround against the vulnerability then I'd argue it's more responsible to publish early and widely than to wait for the vendor.
It becomes greyer if there is no workaround. I'm not sure what I'd support in that case.
Calling what you describe as "Responsible" is intellectually dishonest.
I'd much rather that those things which are remotely exploitable across millions of devices to be kept quite for a small period of time (30-90 days depending on the complexity of the fix required) so that I can get patches from our vendors and schedule an update at the first available opportunity.
You might call it security through obscurity, I call it keeping shit from burning down.
Not telling the world about it does not keep shit from burning down. Eliminating vulnerable targets is the only thing that does that. And that is more expediently served by full and prompt disclosure.
If companies want more responsible disclosure they should introduce harder to find bugs - sneaky edge cases in memory allocation sequences, stuff you'd have to pore over a disassembler for weeks, or slightly weakened PRNGs that would take some serious knowledge of finite fields to discover :-)