I agree with NitroBobby. Also, imagine what happens when a snall pebble gets between the teeth with metal gears.....it could lock up and then the engine could pack it in, unless the clutch gives first. With palstic, the pebble will get embedded in the plastic and continue to rotate, or teeth may break....either way, the engine stays OK.

I used a Robinson CB/spur matched set......they actually wore out...turned out an engine support bolt broke and when teh engine torqued hard, the gears went together (i.e. zero mesh) and the teeth just wore away...little long pointy teeth....LOL. Too much money for that to happen again, so I use plastic now (again........)