"just for some stupid trading" is one of the underlying forces beneath the success of all modern economies. Without it almost everything you buy would be more expensive.
That argument can apply to any improvement in efficiency of a process.
Admittedly any contribution of HFT to economic success (other than that of the traders themselves, of course) is probably fairly marginal, but more efficient markets are probably a good thing.
you've grown so accustomed to modern conveniences that you don't realize that you're kept alive by things that not even a king could've afforded 100 years ago.
Yes, when everything becomes cheaper we can pursue greater technology. Having a 5GHz computer would have been impossible some decades ago; now they can be used for scientific research by anyone who has $500.
Ultimately price is just a measurement of how many resources it takes to make something, with the currency added in as the medium of exchange. A product being cheap means it is now easy for us to make; the previous price of the now-cheap product will be taken up by a better product.
You can if you're a contractor or running your own business.
But it would definitely be nice for businesses in general and employment law to stop being obsessed with 40 hours as a magical number. Working 4 8-hour days, or 5 6-hour days, or 4 6-hour days should be just be an option by default.
(And wages have to stop stagnating relative to prices so that the premise of "cheaper stuff" actually happens in the first place.)