You can always use them if you have to as well.
I use them to stuff my boarding passes, ticket jackets, napkins, etc. to allow easier organization of memorabilia from my flights in my collection.
Back to safety cards, these cartoon characters crack me up! I found a guy that looks... just... wierd, a baby in the brace position in a car seat who has no clue what's going on, Mahmoud Achmedinnijad with a broken egg for a bald spot (later fixed in the Canadian Helicopters safety card; it now has gradient, a small child who goes from little toddler wearing overalls to a Chinese mafia member pickin' a fight in a yellow life vest, Waldo, Glenn Beck, storage bins in the cabin for fuel, fishing bait, and canned food, Gone with the Wind on a TV set I actually own (and it still works!), an iPod Nano, a kid with a severe neck problem, and a guy reading a safety card for a different helicopter than featured on the card! And of course, there's the woman/girl combo in the Northwest safety cards when they did photos holding seat cushions going from smiling and happy to frowning and worried with the plane tilting and them slacking on the straps (what's this supposed to mean, anyway?!).
Is it illegal to take sick bags from the pocket?
Also does anyone ever take the magazines like SkyMall and the airline's in-flight entertainment mags? Those usually have a header that says "Your free copy, feel free to take with you after the flight." or something to that effect. I've taken a couple of those (mainly on Southwest) to utilize the routemaps offered in them.
Why would it be illegal to take the barf bags? They're there for you to use and dispose! I don't think the airline wants you to leave your vomit-filled bag for the next customer to save a few pennies
As for inflight magazines, HECK YES! I book my tickets purposely to get two different months, I'm so bad! Midwest's been slacking there, though, and it seems as though they're getting rid of 'em or just don't care to replenish them; I never got a single one on any of my flights last summer. I found a few from the early '00s, including Northwest's 75th anniversary issue (heavily used, unfortunately) and it's amazing how fleets have changed ever since those magazines were printed, and even terminals! US Airways still had all of A, B, and E, and the end of C. Now, the doglegs of A and B are cut off, E is part-security checkpoint, part-gone, and the end of C is USA3000 and still international arrivals, and A is pure-Southwest in the few gates they have.