Stress and electrolysis
I like using bails; I use a bail on the boom and one on the mast for my boom vang. You can use one for your mainsheet too. If properly attached, the bail is a sturdy piece of equipment. Use self-tapping screws or rivets to attach to the boom. If you are using screws, be careful of electrolysis. Bails and screws are steel and your boom is probably aluminium. Let the two touch, and you create a battery that will eventually lead to the disintegration of both. Coat the screws with some type of protectant before attaching the bails. I have seen at least one product advertised (I don't remember the name) that is made especially for the marine environment and attaching dissimilar metals. I used some standard anti-seize compound, and I have had no electrolysis in two years. Using washers to help spread the load is always a good idea; however, I would put a piece of neoprene rubber in between the washers and the boom. I cut up an old Croakie and made neoprene washers for my bails.