Even hitting stumps and rocks, IMHO a light duty cheap rotary mower is probably all most of us need unless we are planning to use the mower to bulldoze down trees. I have had a variety of mowers of the light duty, el cheapo type and never damaged the gear box or tractor using only the shear pin model. I think the heavy duty decks will last longer because they won't rust out as quick if they are made of heavier guage metal and they are left to sit out for long periods of time. This is generally what goes in my opinion (the deck) rather than the gear box. Even then, you can weld some additional plates on and it will cost you less than getting the more expensive mower to begin with. I have had it done with a mower that was almost rusting through and dangerous and then sold it in still good working order.
Most of the better quality mowers cost twice what the lighter duty ones cost and yet most of us won't wear out the first one, let alone two.
