When high-frequency traders (HFTs) enter markets, the bid-ask spread declines. Several academic studies have reported such a result. Investors pay less for each market order they send. All good and everyone happy.
Vincent van Kervel and Albert J. Menkveld have done the research and present the facts in a well written paper. There is also the blog by Albert Menkveld This paper is good, and often much needed, factual input for discussions about market structure.