That is a weak argument :). Not really, there is a certain and rather high possibility that common man can buy Tesla cars. Sure, there is a premium price tag now, since we are still chipping the first blocks in making that technology a n economic reality. And that specific case is all the more important, since as you too argue helps to reduce pollution.
"Anything that is profitable and legal is of benefit to society" is a little misleading. The definition of what is "legal" is an evolving thing. We know it is dangerous, in that can cause huge fluctuation in the financial control system without real benefit for the society as a whole except a very small percentage. I am not speaking of a central authority that decides it, but our common collective consciousness that should decide it. And again there is not one thing, which has no obvious negative externalities. But the question is whether the positives overcome the negatives. In this case, for the society as whole there is little benefit.
Exactly, when the very society is built on that idea that everyone can pursue whatever they want, the access to opportunities should be equal. HFTs are tilting the balance in one way (helping the rich getting richer) , can cause economic crashes, which hit the rich and poor alike. Now the aftereffects of crashes on the rich (except in case of exceptions) are not as tragic, as in case of the poor. For the poorer part of the society, it pulls the rug under them.