Believe it or not, the springs actually lose their ability to recover from deflection, and in some cases can even break if they become worn and are not replaced. I have seen it more than once. If you already have a spring compressor around the springs it just makes sense to replace them.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280