How do you fill, or refill your hydration system without contamination. I guess they would probably be "one use items" especially if particulate agents settled on your gear. Most wont have a decontamination area to recycle canteens, or hydration bladders, and no way to test for what agent might be present.
The standard military explanation would be that you will have "done your duty" by then, ha. Most of those systems were not designed for use for more than brief duration. I think the drink tube attachment was an afterthought, and not very practical. They may work, in short term drills, and only if there is a guy to tell you when to put it on and take it off.
In the event you do not see the agent dispersed, and don't have the test kit to know when and what was be released, you are stuck living with the thing attached to your face, like an alien life form. Ever try to sleep wearing one, pretty darned miserable, if you do actually do get any sleep.
The mask would probably be good if you needed something better than N95 or N10 masks for short exposure in the event of a flu pandemic, but not the sort of thing I would worry about if there were agents released.