If you use marksdwarves for reserve troops (i.e. not permanently training) then it makes a tonne of sense to give them mostly leather armour and leather or wooden shields. Why? Because when they aren't fighting, they have to haul and stuff. Putting them in full metal will slow them down quite a lot, and there's not much point in making them generally inefficient just for the sake of protecting them from attacks which they shouldn't be subjected to in the first place.
A dwarf who isn't trained in weapon skill, fighting and shield user, simply can't survive in a real battle. Your archers are not going to have those skills (well I guess unless you're cheap and danger room train them). If they are being attacked by melee invaders, they will die. So basically if the fight is easy, they'll just peg the enemy with a bolt and it'll run away or be crippled by pain. If it's a hard battle they're screwed whether they have armour or not because they have no defensive skills.
With that said, as a compromise, gauntlets, high boots and helmets don't weigh much, so you may as well give them those made of metal. A mail shirt is heavy, but, combined with the previous items, it gives full body coverage (a mail shirt covers the upper legs, and high boots cover the lower legs, and it's the same with the arms). Add leather armour and leather trousers, and they have full body metal coverage, without the significant weight of a breastplate, greaves/leggings and metal shield.