Paying bollo is not the main reason. Bollo is around 100-200€ a year. Third Party Insurance for cars is mandatory and in Naples, due to high level of fraud, it can amount to 1500-2000€ a year. Using a Polish insurance, or a fake Polish insurance, is the main reason to get a Polish license plate.

Luckily the loop hole is being closed by the Polish authorities after it became popular.