Bucklers to my knowledge allow none of those things in the current version - they and shields are used in whatever is considered the "off-hand" for the unit at the time. Having a shield and weapon in one hand or with a great-weapon that requires the unit to equip it as a multi-grasp weapon also appears to inflict an attack penalty; enemy soldiers using halberds for example, seem to attack much more slowly than battle axe wielding counterparts when wielding a shield.
I have only ever seen exactly one instance of a dwarf using multiple shields and weapons effectively, and they seemed to have somehow gotten affixed to his forearms - no other units did that and I don't know how it happened, but it allowed him to go axes akimbo on some spawn of holistic very effectively.
Main benefit would be in material and weight savings (metal only, wood and leather shields/bucklers, especially good quality ones in my experience, get used to whack things so often that they go through a shield or buckler in every battle.)
- Marksdorfs would benefit from the reduced weight with an increased rate of fire (slightly, I know encumbrance partially affects ranged units' rate of fire.) You offer them added defense for less impact on their effectiveness (I personally don't bother, since I picture crossbows needing two hands to load and fire and if marksdwarves are in a position to need a shield, they have bigger problems going on.)
- Melee fighters benefit from reduced encumbrance, trading defense for agility. Shields are the inverse, encumbering them more but offering better defense to make up for it. Bucklers may make for better trainer shields until your dudes have thier endurance and strength ground up, at which point switching them to shields might be more advisable, since they can then handle the extra weight better.
- Stockpiles benefit from reduced material cost, allowing you to outfit more dwarves with extra defense for less material, though at the cost of reduced effectiveness.
- If you have an armorer who likes bucklers, then you'll get better results from bucklers than shields, as they'll produce masterwork bucklers much more frequently than they will shields.
- As an addendum, units seem to like abusing their shields offensively, so masterwork modifiers on bucklers or shields (whichever your armorer prefers,) would help them in that regard as well.
These are not science, but observations from someone who uses primarily moderately armored infantry to deal with... Everything, basically, so I have quite a bit of experience in equipping troops, and double and triple checking thier inventories to make sure the idiots have the equipment they're supposed to, as well as carefully monitoring combat logs.