ZOMG a chainmaille discussion! Being someone who actually makes it i think I'm qualified to join in. FUNK is right, Chainmaille is designed to protect against slashing.
"Goodness... the way people keep talking I am starting to seriously wonder what the point of Chainmail is."
In DF right now, yes you are right, but in real life chainmaille is flexible where plate is not. Which is why even when plate became very popular they would still wear chainmaille underneath to protect the joints.
As to the arrow vs. maille discussion:
It depends on a number of factors, like material quality, ring size(AR) and the closures.
Quality is....well quality, if you have it made out of like pigiron or some crap like that your maille wont stop anything.
Ring size plays an important roll as does the AR (Aspect Ratio which is the ratio between the inner diameter of each ring to the wire diameter.) High ARs result is maille that is very loose where the opposite is true for a Low AR.
There are 2 types of closures for historical maille. They are butted rings and riveted. Butted maille is maille where each ring is "closed" by butting the ends together. You would be very lucky to have butted maille stop an arrow as there is nothing to prevent the rings from being pushed apart as the arrow impacts. Riveted maille is where each ring is riveted shut, buy having the ends of each ring flattened, then punching a hole through them and riveting them. This can be supplemented in strength by replacing every other row of rings with solid rings. That is, ones without closures. Riveted maille is much much stronger than butted maille in every regard, but it also takes about twice as long to make. Now an arrow hitting a riveted ring with an arrow there is still a chance the rivet can break and split the ring apart. But an arrow hitting a solid ring would likely stop an arrow depending on how much energy it still has.
All that being said, maille was never arrow proof or anything like that.
Also, adamantine chainmaille would be INCREDIBLY difficult to make, because of its hardness. It would make the chance of a ring breaking when you bend it shut and if it doesn't break the ring will likely be warped which would compromise the strength of the maille.
Now, questions?