My hingeallelogram (HGL) allowed for two inches of vertical movement (see comment image: http://www.instructables.com/id/DIY-High-Speed-Book-Scanner-from-Trash-and-Cheap-C/?&sort=NEWEST&limit=50&offset=100
I never tried the system without it. It allowed for several books that I scanned, and I think it worked well. I also think the two inches of vertical movement can accommodate for a large set of books. In theory, the two inch rise should accommodate for a book that is as much as four inches thick.
Why two inches, when the full range is closer to four inches? Well, if you notice from pictures I had put an "L" bracket to stop HGL from full range of motion. If I allowed a full range of motion, then HGL would not expand and then collapse to the other side. By keeping freezing the HGL to its max extension, it can only go back one way.
When I first was raising the platen, I notice that the HGL would not contract until raising past a certain angle (say 45 degrees), in which the HGL would collapse causing a sudden movement/fall of the raised platen. I added the spring to stimulate the contraction of HGL early to prevent the sudden movement.
I know several community members questioned its use/function; however, it worked for me...
As for function, it worked very well.
As for possible caveat:
As the HGL raises, your platen's depth changes. That is, on page 1 of the text book, the platen is the farthest distance from your chest (the collapsed HGL); and in middle of the book, the platen is closest distance to your chest (the expanded HGL). This depth movement is about 2 inches (the same distance as raised); which is not really a concern.
I don't know in regards to other scanners, but I found my book sliding up (away from me); which is why I needed a way to secure the book in one place (I used twine to strap the book).