Yeah it's not that simple, wikipedia has a good summary:

Advocates of final destruction maintain that there is no longer any valid rationale for retaining the samples, which pose the hazard of escaping the laboratories, while opponents of destruction maintain that the samples may still be of value to scientific research, especially since variants of the smallpox virus may still exist in the natural world and thus present the possibility of the disease re-emerging in the future or being used as a bio-weapon.