I can't help but be reminded of that Taibbi quote about Wall Street being a vampire squid plunging its blood funnel into anything that smells like money. It's a living, I suppose, but does "running your arbs" create anything of value?
Something similar could be said of the people chasing the ball around the field.
The process of arbitrage involves observing a mis-priced asset and then making a profit from that observation. By doing so, you push the asset towards its correct value. This kind of activity maintains the integrity of a market.