I gave it some more thought as I lay trying to sleep and here are my thoughts:
There is very little we cannot do with robotics today. Here on Earth we use robotics to manipulate the very small as well as the very large. Robots can handle materials in a very wide temperature range. Though environment is an issue for both human and machine, I think machine holds the lead here as well. The extremes that we can achieve with robotics far exceed human capabilities.
Why send a machine to do what a man can do? I think that if we look back through human history we can see that many human lives were sacrificed to tasks we can achieve today with robotics. An example that came to mind is the construction of the railway system across Canada. Specifically, I considered the construction of a section of line near where I live in BC. Many lives were lost due to extreme terrain. Mining is another historic activity that has claimed many and yet we continue to sacrifice more humans to this industry. Deep sea fishing, construction the list goes on and on. All of these tasks could be accomplished with robots.
This brings me to the human issues. If we have robots do all this what will we do? What of the statement " ...humans want to go(or do)..."?
First, the what will we do ... question. I'm not going to touch that one because the answer involves totally rebuilding our society and I do not see this happening here on Earth for a very long time.
Second, humans want to.. do things. That's great, go ahead and do things you enjoy and are passionate about. I think there is a difference between doing something because you want to and doing it because it must be done. For example, if you want to jump out of a plane because you enjoy the thrill that's okay but if you are doing it to go fight a forest fire then why are we not sending an expendable machine. If I lost a friend to sky diving, I could comfort myself in thinking he died doing something he enjoyed rather than his job killed him.
So, do we send humans to the moon or Mars or asteroid mining or deep space missions? If the mission is to get information or material then there is no need for humans. Human missions should be strictly for the progression of human expansion or colonization. If we send humans to live on Mars or to visit Mars as a tourist and if they "explore" in the process, that is acceptable. We have attained a level of technology that could allows us to eliminate menial tasks from the human experience. When these menial tasks involve significant risk then Machine should be employed. Going to Mars and picking up rocks and analyzing them is definitely a machine task. If man wants to go to Mars they should do so because they want to and not for science or occupation.