You're right, it isn't necessary but it sure is worth considering since distileld water isn't that expensive. For a couple of quid every two years you should extend the life of your radiator (and run your engine cooler) considerably. In truth, I do not drink tap water because of the amount of crap I usually find in it. Studies show that water in my are is so bad that it has benn known to trigger allergies and even loss of hair in men!!! I think I won't subject my car to that!!!
In hot countries you don't really need antifreeze since obviously you do not drive in freezing conditions. Antifreeze though usually contains detergents which ensure that the cooling system will run at its best. Antifreeze contains a chemical formulation which - when mixed with water - it allows this to remain in liquid form even under -50deg C. The higher the concentration of antifreeze, the colder it will need to be for the liquid to freeze. In your case I would still recommend using antifreeze but dilute it with more water e.g. 30parts of antifreeze to 70parts of water.