Mortality is not the only useful measure here. It's great that fewer people die with or without vaccination, but there are impacts from being sick that are also useful to mitigate. In the worst cases, people can become permanently debilitated as a consequence of a disease that in others just caused a nasty rash. Another facto to consider, and this IMHO is compatible with the interest in military use for some diseases, is that if a big number of people gets sick at the same time, it is highly disruptive of life in society.
If instead of waiting for people to catch diseases and then gain natural immunity you do something to acquire immunity, you have a better chance of avoiding big numbers in morbidity, not just mortality. It is not just a nice bonus if by doing this each individual is less likely to be spreading the disease for several weeks that year, and is hopefully not becoming deaf or blind.