Coming from an ex Autozone parts salesman, they do have their benefits. They kind of absorb the corrosion. They're particularly good for those terminals that no matter how hard you try they won't set flush against the base, therefore creating a gap. They are a cheap investment for those of us who don't break out the baking soda often enough. I've used it all and baking soda is the best thing I've ever used. I personally hate putting dielectric grease on battery terminals (tho I was required to do so) seems to not have as good of a connection and the terminals work their way loose. Grease is great for screw in terminals like in gm cars. I've never used the protective spray but have seen good results with it.
All in all I think they're worth the $0.99. These have no negative effects that I've found.