When high-frequency traders (HFTs) enter markets, the bid-ask spread declines. Several academic studies have reported such 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.