What are you going to do if you accidently hit a rock while trimming and it goes through a window or sliding glass door? The cost to replace these will cost you more than a couple of years worth of general liabilty insurance premiums will. How about hitting a rock and chipping the paint on a customers car? A quality repair may cost more than the insurance would have. Do you think the customer will let you have your "buddy" who will cut you a deal fix it? More than likely they are going to want it fixed by someone expensive. How about hitting a rock and having it hit a kid walking buy on the sidewalk? Can you afford to pay the medical bills? Because God forbid it was to happen here is what will go down. His medical insurance will find out he got hit by a rock thrown from a lawn mower, they will pass the blame onto the homeowner and their homeowner's insurance. The homeowner and their insurance are not going to pay for it, they are going to pass it right onto you. If you don't have general Liability you will be stuck paying that bill for a long time. Think it can't happen to you? The happen all the time. An even worse scenario, what about a lawsuit if the rock to the head kills the kid?
Does your state require commercial auto insurance and commercial registration? Even if the state doesn't require it, will your auto insurance cover your vehicle if it is being used for commercial purposes. Most insurance companies will deny the claim. I have told the story here of a guy I know that lost control of his personally insured truck in a rainstorm and crashed into a lexus dealership. He caused hundreds of thousands of dollars in damage. The police saw his equipment and put 2+2 together and put in their report that he had landscape equipment. Insurance denied his claim and he was sued by the dealership, this then threw up red flags and he was audited by the IRS. He ended up loosing everything and him and his wife ended up on probation for tax fraud.