Falcon 9 does have engine out capability and that's been proven in the past on CRS-1. In theory even if one of the engines were to rupture the shrapnel should b

Falcon 9 does have engine out capability and that's been proven in the past on CRS-1. In theory even if one of the engines were to rupture the shrapnel should be contained by walls between the engines. Having less engines per core would mean an engine out condition is much harder to recover from as you would lose more thrust. If an engine goes out you don't just lose that single engine, you need to make sure that the thrust vector is still pointed straight at the center of mass. If you don't, then you'll get torque on the vehicle that will make the flight end sooner rather than later. If you plan for engine failure to not kill your primary payload you need to be able to use a combination of thrust vectoring and gimbaling engines and still be able to get the payload to an acceptable orbit with the reduced capacity. More engines means more chances that one of them will fail but less severe consequences when that failure occurs.