it's fucking stupid
if a chemical/biological attack occurs, it will likely be in a transportation center such as an airport, subway system, or train hub. Taping plastic over your window will do nothing, as it most likely won't get anywhere near your house anyway.
on the off chance that a chemical/biological attack occurs on a broad enough scale to affect residences in a city, taping plastic over your windows will do NOTHING!
let me point out how many ways air gets into a house, and how it doesn't.
winterized windows, are for the most part airtight. what good does it do to put another layer on them? Dryer vents. cooking stove vents. heat convection through leaky attics. the crack under your front door. this is where most of the leaks in a house are, not to mention through cracks in floor boards, and all that fun stuff.
this is just like sticking your head in a paper bag to prevent the aliens from getting you.
placebo for the masses.
don't be duped. but also, don't be stupid. if an attack occurs, block off what you can. but don't fall for the concept of there being one way to do so.