It depends on where you live. I recently had to replace both the resonator and muffler on a '91 300E because the weld going into the muffler had rusted through. Except for the weld, the exhaust system was fine. This car had spent 10 years in Minnesota.
I suppose I could have had it welded but by the time I discovered how bad it was, it was too late. The original exhaust system is one piece after the catalytic converter. The MB replacement was in two pieces.
I now live in Arizona and expect the exhaust to last forever.