Usually they spin, the outer part turns a bit from the inner and it becomes impossible to set your timing well as the marks are off, plus you never know what you may break if your engine is shaking too much....engine mounts...premature bearing failure etc etc...not worth running with imho, I paid 100 for a decent balancer, took 30 minutes to put on, made a huge difference.
Don't run a bad balancer. Vibration at high rpms will be a killer on your bearings. Not worth having to tear into your engine down the road because of failure because you left a bad balancer on it.