Logistically, it's asking for trouble. If you can't park the bike safely within the customer's premises, you massively increase the risk of theft or damage. If the nearest immovable object to lock to is 100 yards away and it takes you two trips back and forth to get your gear to the customer, some of it could get nicked even in the time it takes.
People have made comparisons with other jobs like Paramedics. The difference here is the expensive equipment does not belong to them, and if it gets stolen or damaged at work, their employer is going to pick up the tab not them. Also, they still have to be paid even if they don't have their equipment. If a self-employed person cant do their job because they don't have the gear they need, they aren't earning any money. Big difference that those who get over-enthusiastic about the idea of carting tools around by bike should think about.