I am pretty sure that I am the one that started the "bag mania" around our families. The bag that started it all is the orange one on the right. I've had it for many years now. It was a bag that I copied from a bag I saw in the library one day when we were there for a story time, when L. was 4 or 5 years old I believe. I believe I discreetly followed this lady around the library to get a better look at the bag and make a few notes on a piece of paper so I could reproduce it later. It had this cute little side zippered pocket for the library card and if I remember correctly it zipped close at the top too. Then her name or her daughter's name was embroidered on the other side.
I didn't want to put a zipper in the bag so our "Library Card" pocked is velcro'd. Much easier to install and will not break or catch, etc. All great things when you are dealing with kids. These bags can hold a TON of books and then it also helps to keep all the library's books in one place so that they are less likely to get lost. And you don't have to keep swapping the stuff out of your other bags when it is time to go to the library.
Over the years we've made a number of these for friends and family. The latest one is the brown one on the left. Z. felt he needed his own, and one that wasn't too "girly" looking. So I think this makes for the 4th one in our family alone. However, instead of putting "BOOK BAG" on his we put "READING ROCKS". He is very happy with it as he got to pick out the inside lining fabric and what it said on it. Inside lining fabric.
I am all for doing whatever you may need to in order to get your kids motivated and happy about reading. If that means they each get their own library bag and card then so be it. READ ON!