Why do you need to maintain manufacturer given clearance limit for Main bearings in a Marine engine?

If the clearance is too small it may lead to wiping of white metal bearing,shorter life span of bearing shell,,thus frequent replacement of bearing shell,

If the clearance is too big,no lube oil retention to cushion friction part of crankshaft and bearing shell,much faster bearing shell failure.could lead to whole crankshaft failure due to overheating as no lube oil.

Correct clearance to keep cost at low side.

The Clearances as defined by Maker should be followed strictly if not followed the rotating parts could damage the Crankshaft, resulting in more down time and cost of repairs. SO better to replace a Main Bearing than the entire crankshaft.