i think when someone laments that a high-frequency-trader/hedge fund managers don't add value, they really mean that they get an unfair amount of money for little visible work, and little visible benefit to somebody else in the world. I emphasis visible.

I do believe that having arbitrage evens out the pricing of long term assets, and having traders do this continuously does indeed make the prices of things reflect reality.

That they make a lot of money doing this seems unfair, but the world isn't fair.