Perhaps I am wrong about the drier.... but an a/c guy once told me that no matter how sealed the system is, small molecules get in, and small ones get out....

and the dessicant (spelling?) the drying agent gets used up.... at what rate I don't know... I have never changed out a drier just for the sake of doing one.....

I am told it is moisture that ruins things like the evaporator... of course at some point the hoses will disintegrate...and the mechanics of a compressor will fail.

maybe we need an A/C guy to weigh in....

otherwise, what would you need the drier for in the first place?