It is the glass that forms the strength of the panel. With that broken, the cells, if not already cracked can flex with the eva and tedlar they are encapsulated with, causing them to crack easily. I have a couple of broken panels and have been meaning to try and do something with them as well. My thoughts were to cast some resin on the back of the panels inside the frame to limit that flexing, and to give a skim coat of uv stable epoxy on the front. I don't think that would work for the larger panels like you have very well though because the flexing would still likely be significant. Once the cells are cracked up, even if there is still a series connection and voltage, the panel will not put out any current to speak of and is just scrap at that point.
