Altruistic behavior often leads to desirable social outcomes. We can thus assume that more altruism is better than less, other things being equal. But altruism tends to be already widely encouraged, so efforts to promote it even further may produce little noticeable change. Instead, it might be easier to do more good by improving efficiency of the altruistic behaviors already in place.
Jonathan Baron & Ewa Szymanska, Heuristics and Biases in Charity, in Daniel Oppenheimer & Christopher Olivola (eds.), The Science of Giving: Experimental Approaches to the Study of Charity, New York, 2011, p. 215