Spun rod bearing- What caused it?

[ROLLGUY]

I finally got the time to tear into the engine that was in my first 'Benz ('82 300CD). I replaced the engine with a good used one a couple thousand miles after I got the car. It developed a bad rod knock. #2 & 3 con rod bearings were spun down very thin. In the photo you can see the bad bearings:


The crank is bad too:


Notice the pitting in the main bearings:


The bores look real good, so I will save the block. Is the crank worth saving?