the idea to go leashless came from the first ice comps where climbers would pump out and fall off and hang from a well placed tool by the leash, dangling in space, unable to go up or down or reach again the tool from which they were suspended. in this sense it is not unlike an etrier attached to your tool which in the Canadian Rockies they used in former time.
i agree that on multipitch alpine the leash may be more necessary to prevent dropping because it could be serious if you were to lose a tool. but these routes have also different game rules so perhaps it is only at ice crags that the leash should not be used?