They are afraid and double minded about what theyre doing and so they do it half heartedly. Thats what I think anyway. If you were confident and sure about what you were doing then you would do it firmly and properly.
Although it may make it unstable and therefore make it a stability excersise, but not on a bloody machine, its hard enough to activate Core and stabalising muscles on bloody unstable core exercises with unstable equipment, usually people send the force into the joints.
In my experience the noise of rattling plates can motivate you when youre on your own.
But still I give it a definite