Well, the counterweights are there for a purpose so without the correct one your PC may vibrate a lot more than normal, but I wouldn't know first hand as I've had the correct one from the get-go. Short-term I'd imagine the only problem you should have would be the extra annoyance of the machine vibration, but long-term, being out of balance could cause damage to the shaft and bearings.
Since I don't have first-hand knowledge of it, I wouldn't know the actual severity of any possible damage, but I'm sure someone on here should be able to give you some conclusive input.
