Pro- fish feels safer
-don't have to worry about seeing ugly cords
-depending on what wallpaper you use, it could remove reflections in the glass that bettas sometimes flare at
Cons- if looking for the fish you can't go to the back off the tank to see if he is hiding somewhere, the wall paper will block your view.
-you need the glue stuff to get it to work properly, and it is gross and sticky lol
That is all I can think of right now. I have a wall paper on my pea puffer's tank, it makes him feel safer since the back isn't "open" to anything. I can take a picture of it if you want, as an example.