This (rod bearings) is a very common issue on the V6 SHO engine with which I am quite familiar.
It has become standard practice among SHO owners to replace only the rod bearings as the mains do not get the same type of stress during operations as do the rod bearngs. The rod bearings take stess with every firing of the cylinder and are the first to go.
Main bearings can be replaced without pulling the engine, but it's pretty tricky to get the crankshaft to drop a bit in order to swap them out.
Based solely on my experience with the V6 Yamaha, I would not worry or fret about the lack of replacement of the mains.
Steve
