Umm, nope, there's absolutely nothing on asteroids that's valuable enough to justify mining them in near future. Even being made of solid platinum or diamonds wouldn't make it profitable. Everything that can be gathered from asteroids can be mined from, for example, the Antarctic or the floor of the ocean which are both simpler and cheaper to reach.
Asteroids and comets will be valuable in so far as they contain plentiful minerals that are already in space. If people want to build bigger things in space then having available materials already in space to build them with would be valuable, and it would (probably/eventually) be cheaper than flying them up. Not to mention more environmentally friendly as mineral processing is a pollution intensive industry.
Sure, but it never happens unless you do these early experiments to find out how e.g. to dock with a comet/asteroid, to attach, what energy budget you expect to need once you're there and so on.
If your goal is to do X in space, then mining asteroids might be a valid means to help you do X, but not an end goal by itself.
Most missions don't need a large supply of water, hydrogen and oxygen located in a random orbit around the sun. If you need them at Mars or Moon, then likely it's easier to find the supplies there instead of spending a large supply of fuel in order to move a large supply of stuff to that orbit.
If we'd need stuff at LEO, it might be that flying to an asteroid (generally far, far away from LEO) and then pushing it to LEO is more efficient than pushing it up from Earth, but it's not so clear.
In any case, this scenario of mining asteroids is very different, I somehow think that the grandparent poster meant mining stuff for us back at Earth.
