If you've got a whole lot of knives you carry around, you probably have a knife roll. Me, most of the time I just need to bring one knife or maybe two. So blade guards make a lot of sense.
The magnetic blade guards ($9-$12) from Bisbell have a couple of advantages over plastic guards. First, they're easy to cut with scissors so you can shorten them to fit specific knives.
Second, they hold the knives snugly, so the edge of the blade isn't bumping around the way it could inside a hard plastic guard. Maybe not a big deal if you're just taking your favorite knife to Aunt Susie's house to carve the turkey once a year, but more important if you use those guards to store knives in a drawer.
Although these things are magnetic on the inside, they don't stick to each other, so you can carry or store them together or with other metal items without worrying about them grabbing onto everything they touch.
While this isn't the most earthshaking gadget I've seen, I have to say it's a nice improvement over the slip-on plastic guards.
The product was provided to me by the manufacturer for the purpose of a review on Serious Eats.