Such a mail shirt indeed does not make much sense. Although the question of sense in a magical high fantasy world may make no sense too. The shirt, if it's an aura, why are magical attacks from near not punished? If the wearer needs to attack, why does the shirt only help you if a melee attack hits you? Perhaps the maker was drunk or had a strange kind of humor?
Generally, my biggest concern with magical items is wether you need them or not. For me often it's a fashion choice. For example the Barbarian gloves giving 15 hp after rage has ended: it gives you also a blueish aura which I find, like all magical effects, extremely ugly. Therefore I sold the gloves asap. I also will never wear boots as Barbarian, or a helmet. If you needed strong items to keep pace with enemies, say on "Normal", that would be bad.
