I will use 1:9 with HP5+ but not for any other film. My experience with DD-X, and it is probably true with other similar developers is that the film grain gets a little more sandy in texture.
Another reason why I use DD-X at 1:4 is because I value my time.
Could the grain be sandy at a dilution of 1:9 because there is not the minimum amount of stock solution in the working solution, i.e. at least 61.5 ml of undiluted DD-X? Shouldn't the grain be sharper at a higher dilution?