Using influenza as a bio-weapon makes as much sense as issuing plutonium knives to soldiers for use in combat.
There are plenty of reasons for this. For example, the various influenza have an extraordinarily high number of flexible RNA components, which means it mutates at a tremendous clip. A single animal may have half a dozen strains at the same time.
This also means that the strains are in natural selection competition with each other to develop the best strain to insure strain survival. This means that there is a distinct path for a given strain.
To begin with, a strain must be different enough from previous strains so that its hosts immune system does not recognize it and attack it with a specialized and deadly defense. But by not recognizing it, the host immune system overreacts, which can result in what we now call the “cytokine effect”. Between virus over-reproduction that damages the host and the cytokine effect, this makes the new strain deadly to the host.
However, natural selection then kicks in again, as less lethal variants of the strain survive and propagate better than lethal ones. And this happens rapidly as well, so the strain soon becomes just annoying to the host, not deadly. But by then, so many hosts are innovating deadly defenses against the strain, that other strains are able to fill the gap in the host population.
Influenza also swaps RNA segments readily, helping it to adapt and compete. This is why the Mexican swine flu has human, avian, and European and Asian swine elements, and typical influenza tests showed positive for two different strains.
In short, influenza is vastly too complex, too capable, too flexible, and too unpredictable to use as a bio-weapon. Annual vaccinations are based on guesswork, and even if a lethal strain was devised, there is no assurance it will last beyond the first few hosts.
At the same time, the H5N1 strain being innovated naturally has the potential to kill 1 billion people. Nature is that much better at this than we are.
IF you looking for disruption, then it might make sense.
But blowback will be bad, you have an even chance of killing off your own guys. Which is the same reason that chemical weapons still have the stigma they do, and subs don't anymore. To easy to kill off alot of people you weren't aiming at.