Yep - sounds like an arse-covering exercise there by the mechanic. I've never heard of spun big-end bearings on this engine and only one that I can recall on the 4.7L V8 in the Grand Cherokee (same engine with 2 extra cylinders) and this was due to overheating.
As John says, a spun bearing is going to be the result of an oil supply issue (or overheating issue) and not from a bearing/locator design flaw.
I hate when mechanics make shit up!!
