Let's explore the reasoning for using anthrax vaccine.
It is supposed to protect you against anthrax. No studies of the current vaccine have been done in humans to see if it works, and how well it would work. It works in some animals but not others.
But for argument's sake, let's assume it works well. Will an enemy spray troops with anthrax if it knows the troops have all been vaccinated with an effective vaccine? Keep in mind that military doctrine calls for a powerful response to biological attack, which could include nuclear weapons. It's not very likely that an enemy would take such a big chance against effectively vaccinated troops, since there would be very little to gain.
If the enemy was serious about biological weapons, and ready to risk it all, the enemy would instead use a weapon that troops have no defense against, like botulism or Ebola.
This is a key concept: Anthrax vaccine isn't meant to save troops after an attack--its true purpose is to DETER an anthrax attack. Anthrax vaccine is also being given to soldiers to make a statement to Congress and to the world: vaccinations are "proof" that the US is prepared and ready for biological warfare.
But if the truth be known, the US is entirely unprepared for attacks by many types of bioweapons. We have no vaccine and no antibiotics for most viruses, for example, and no antidote for most toxins. Even bacteria can be made antibiotic-resistant.
Carrying a supply of doxycycline or cipro would provide an equal amount of deterrence as anthrax vaccine, with considerably less danger to troops, and at a much lower cost. DOD has been allowed to store most antibiotics for ten years to increase shelf life. This is not an approved option for vaccines, which is why the US will be wasting 100's of millions of dollars on anthrax vaccine that has been stockpiled by DHHS.
It would be better to do the logical, safer thing and have troops carry antibiotics, especially since units now have sensors for anthrax that can provide early warning of an attack. Besides, doxycycline is also a deterrent against brucella, plague, mycoplasma and some other biological weapons for which we have no effective vaccines.
It would also be better to acknowledge that we remain unprotected against a variety of chemical and biological weapons, and deal with this fact, since it is unlikely to change any time soon.