Rusted filter drier are often written as a deficiency in a home inspection report. Normally once rust start is does not stop. You can slow the process but eventually it will rust through. Drier Filter filters out moisture and small particle debris within the cooling Freon system. It is normally exposed to the outside therefore it has a chance to corrode. Once the drier has rusted through it will leak Freon. Sometime very slow and not noticeable. You will your home not cool as fast or not cool at all. A lot of time a service call will cost around $300 after filling your unit up with Freon. Everything is good until Freon leak out again. It is very important that your service technician find the leak. With prices for R-22 going up fast the last thing you want to do is pay a lot of money to temporary fix the problem.