Jethro,
Like everything about tractors, there is no "correct" answer to this question. Also like everything else about tractors, it is a matter of balancing cost today against the probability of saving time and inconvenience in the future. The cost today is about $100 for the slip clutch. The time and inconvenience you may save is if you start hitting unyielding things with your cutter. Unyielding things are: hard woody brush, thick trunks, rocks, old metal fence posts, and, on very hilly or bumpy or potholey ground, the ground itself. If you start banging into these things and break a pin, you have to get off the tractor and reinstall a new pin. The more times this happens, the more time and incovenience of fixing it. If you have a slipclutch, you dont have to do anything; just keep on driving.
So, the first question is: do you have the kind of terrain where you will be hitting unyielding things. If the answer is no--eg, you are just going to be cutting long grasses or light brush on flat land--then it's probably not worth spending $100. Many people who do this kind of light brush hogging will tell you they have never broken a pin. However, if you have unyielding things on your property, you have to predict how often you might break a pin.
I bought a medium duty cutter with a slip clutch. It was worthwihile because I have slipped the clutch many times on my hilly, woody, rocky New England property.
Of course, the ultimate kicker is that once you have cleared off all your unyielding things, you wont need the slipclutch anymore and then you can worry about it rusting up.
