Yes, it needs to be replaced for system efficiency and longevity. While you might not think so, as far as the a/c system is concerned, is tremendous amounts of moisture in the atmospheric air. This moisture can not get into (or remain in) the inside of the a/c system because 1) water and air are not compressible (which causes poor a/c performance) (o.k., air IS compressible, but it takes up space that should be used for refrigerant, which is what causes the cooling effect); and 2) the moisture mixing with the oil and refrigerant can create acid, which will damage the inside of the components. There may be other consequences of moisture, but in my opinion these two are bad enough. You need a new drier and a thorough evacuation every time the system is open.

Question: If you intend on keeping your cars for a long time, why are you buying cheap rebuilt compressors. In the long run, and even in the SHORT run, you are spending way more money. Plus, if one of these rebuilt compressors blows up and throws particles throughout your system, you really caused yourself a lot of work and financial expense.

GregS
'84 300D, 176k
'90 300CE, 167k