Asteroids not only contain iron, nickel, and other minerals such as those, but they contain gold and platinum, some of the most valuable minerals! NASA plans to send a robot into space to mine asteroids, so it isn't at all dangerous to humans. However, it does indeed cost a lot..
Asteroids are a very good source of iron as most are almost completely made of them. This would be hard yes but, we could obtain minerals from an almost endless source while here on Earth we have only a limited supply of ores and minerals. It would be hard to dock to an asteroid, have smelting plants, have a legitimate workplace, and it would be VERY costly but it is indeed possible.
Yes it should be. It would find new elements and may be it discover the new theory, new facts and new beauty of space. It would only benefit us giving us simply new facts. May be it cause a new revolution in the new science world. Just keep waiting for that!!
If we mine asteroids, we should do so far enough from our planet so we avoid the danger of an asteroid changing orbits and crashing on our planet.
Apart of that, it is not necessary, as we can get the minerals we need from our own planet.
And expensive. It is cheaper to just do normal mining.
Asteroids are continuously moving so technically it would be impossible at this time to mine an asteroid. Not to mention the time it would take to collect and gather the minerals and to bring them back to Earth. It would also take up too many resources just to send machinery into space and back. In theory, it would be cool but its not efficient in today's world