It is still an effective weapon until we get enough vaccine to stop a widespread outbreak. But, even then it is problematical as the Soviets have supposedly developed species resistant to our typical smallpox vaccines. If a terrorist organization had some of these bugs, all the vaccinations in the world wouldn't save you. Remember, almost 95% of the pre-Columbus Amerindian population was wiped out by disease to which they had no immunity. The survivors and their defunct civilizations were devastated. They went from a nearly unbeatable power to slavery in a few short years.
There are lots of biologicals other than smallpox that are a threat. Sure, they may wipe out half the world. But, do you think Al Queda would care? It's the cause that counts!