I have your very same Yanmar 5' tiller (I think). To answer the question, the slip clutch is recommended in case your tiller discovers a rock, a cable, a chain or breaks the chain case and siezes up instantly. The slip clutch frictino will be overcome and the
pto will still be able to turn, but under heavy load and some engine strain. You will know something happened, but the jolt will probably not break the gears, or twist the shafts, or let you continue with motion but no tilling. Because its a slip clutch instead of a shear pin, the powertrain will still be solid, so, if you run over a rock and the clutch slips, you could pass by the rock and the tiller would continue. All you would have is a skip in the garden. With a shear pin, its end of session.