The problem is once you are in the system, you never truly leave. You de-register but you still owe your dues and demands in Belgium. They will continue to accrue for as many as 3 more years. Then when you come back and re-register, your history will be there to haunt you.
However, it is a very interesting point you raise, especially for non Belgian nationals who drop in and out of Belgium over the years on different contracts, with periods of de-registration (assuming they were properley registered each time ). I'll ask an expert and write more once I get an official answer.