Recently bought some dollar books and they were bagged and boarded with the board in the middle of the book, instead of in the back in the bag. The book did feel more stable this way, but it leaves the spine corners exposed to dings.
So, I left the board in the middle, but rebagged it with a board in the back too... I was very pleased with the result. The books are very sturdy, and stack level .
Was thinking of doing this to all my bronze age books and keys. Any reason not to do this? Someone mentioned that they feel this would increase the probability of spine ticks. I'm not sure how, as inserting a board into the book reinforces the spine. They place boards in the center during pressing for that very reason.