Well, that's the second front to the question: Should under-represented groups be encouraged to fill historically gender/racially-biased jobs? I have a feeling that few will argue about it in the case of coal mining, but you definitely see it in the fields of science. I have yet to see any contention over those movements, though. Occasionally, hacker news has a discussion about women in computing fields, but usually they are about how to promote interest in the industry (As well as combat sexism within it), not whether to. The lack of any discussion about a particular job, such as coal mining, probably reflects it being an unfavorable job that no one wants, but unfortunately a few find themselves doing for lack of a better option.
Yeah, people usually only talk about people getting into jobs and positions that are highly sought after, so everyone wants to see more women in science and everyone thought it was fantastic that a black man became US president but no one seems to say "hey, there should be more female garbage men." They're making a barely distinguishable but critical mistake. The point is not that certain groups do get into certain places in society; the point is that the roadblocks preventing them from gaining access are removed. (This is sort of tied in to what humanodon said about balance.) Focusing on getting certain people into certain places is part of the illusion of the whole "glass ceiling" thing. It isn't about numbers. Attitudes toward black people, overall, are probably not going to change significantly just because Barack Obama was elected. Although, in the long term, it probably does make a difference. And it does suggest that at least people are willing to elect a black president, but then that makes it a symptom of changing attitudes, and not a terrifically powerful causal factor. (And, of course, the fact that many would have voted for him because he was black is itself a problem.)