IMHO bioweapons would be more effectively used in the lead up to a planned attack than during an attack. Diseases can be difficult to attach a "return address" to especially if spread clandestinely. If you are trashing your enemies with a full out nuclear attack the effectiveness of bioweapons may actually be diminished because, while they may be very effective in any pocket of survivors they hit, the destruction of transportation links will outweigh the reduced immune response of survivors due to malnutrition and/or radiation exposure. As noted, using bioweapons anywhere near your borders or where your forces might be risks spread in to your own population.
With the exception of spore forming organisms like tetanus and anthrax, disease organisms (virus, bacteria, rickettsia, parasites) need some sort of reservoir whether human or animal. The reason there is no more smallpox is that it only affected humans and when all smallpox in humans was eliminated (in the 1970s) it vanished - except for (hopefully) only two labs where samples are kept. In the wake of a full on nuclear war human to human transmission, human reservoirs, and animal reservoirs could just as well be disrupted not just maintained or augmented.
By using bioweapons against an enemy physically isolated from you (USSR vs USA) in advance of a planned attack, you could seriously weaken the enemy and kick them when they are down. Another issue with diseases is, once they are identified, procedures to prevent spread even if they are difficult to treat, are well understood and straightforward.