Sillypeoples:
Personally I would take a lighter engine, less overbuilt, running in a more efficient operating range to save some fuel...if the fuel savings were such it made sense...but not if engines were breaking left and right...
You are probably quite right regarding efficiency, but then the issue becomes logistics and commonality.
The CFM56-3 started out at the 3B1 rating, then with some hardware upgrades was "pushed" to 3B2 and 3C1 ratings - all within the same carcass, mountings, accessories, etc. If an operator selected the lower thrust rating for his fleet, it would do him little good to insist on a physically smaller engine, when that would cost more for special tooling, spare parts, etc.
Further, if the operator decided to trade up to a newer fleet some years later, the new buyer of the old planes would have the option of using the higher thrust rating if required on his route system.