Some charter boats would have their eng's rebuild/gone thru around the 2500hr mark. Having multi charter boats, this helps spread out maintance/replacement instead of having more than one boat/yr down and costs involved. This also helped reduce chances of an engine failure while on a charter and having pissed off customers and having to be towed in. Just as insurance so to speak. Also this was on carburetor engines. Carb engines run ALOT richer than injected ( at some load points more than triple of an injected engine), the excess fuel gets past the rings, washing down the cyl block walls, dilutes the oil, which increases internal bearing, cam wear dramatically. Just compare average engine life on older carbureted engines vs todays tightly fuel controlled injected engines. Your spark plugs, dist. cap/rotor, appearance of the oil on your dipstick can tell quite a story. Along with oil pressure readings, vacuum gauge, engine performance etc.