2 reasons I use the acid dyes. The first being that it is much easier
(I have done natural dyeing also, but it requires boiling the yarn in a solution of alum, and is more time consuming). Natural dyeing also involves some calculations and weighing, etc., which is okay, but I have to be in a certain mood to do it. Also, I can get a whole range of colors using the commercial dyes!!!!
Also, I do find the colors are richer with the acid dyes, which by the way, don't work for plant fibers. You have to use a fiber reactive dye with them.