This design allows for part of the weight to be distributed as an upward force on the piece of wood closer to the wall. The weight of all the other spool holders should prevent the lever from pulling up that piece of wood. The lever is thinner than the body, so it can easily pass inside the space between a spool and the rack (hopefully even with a full spool). 

To prevent movement from left to right, the body (holding onto the front) is wider preventing torque from the use of the spool hanging to make the holder wobble.That said it is thin enough that can fit between the walls of a spool in order to not upset the rack.

Friction on the spool that prevents it from unravelling for the printer is the only issue I can think of. The edges that would be touching the spool have been rounded to prevent this but I think it will really depend on the material this is printed with. In any case, I left enough space on the top of the holder to place some bearings if it really comes to that.

Cheers!