Smyth sewn: thick/visible glue vs thin/invisible

I am wondering what the difference is between smyth-sewn books that have a visible glue layer vs ones that don't. In particular all of my modern reference books have a nice, thick glue layer on the signatures like this:




However the older reference books in my library don't have a thick glue layer. It is completely invisible, e.g.




To clarify: I have taken apart one of the "thick glue" ones and it does indeed still have horizontal stitching between the signatures. So the glue is not being used to hold the signatures together horizontally.

Also, I have noticed that all of the lower quality/imported smyth-sewn books in my library have a thin/invisible glue layer, whereas the ones from respectable publishing houses that have good quality covers etc all have a thick glue layer like in the top image.

Also, just intuitively it seems like a thick glue layer - all other things being equal - is more durable than a thin one in a smyth-sewn book. But I have received advice telling me otherwise which has left me a bit confused.


Thanks