I have always preferred to use a selection of yarns in the weavings I produce, and for me the mixing of these different wools, cottons and rayons in particular give the weaving a less flattened and more interesting appearance. When trying to present 2-dimensional fibre art as fine art, I like to enhance the texture of the surface, especially as my weavings tend to be pictorial and the images in them easily identifiable.
The use of texture range is important in the weaving if the veiwer is likely to critique it in the same context as a 2-dimensional painting, As much as that, having all those cones of scrappy odds & ends feeds the magpie in me, and gives an excuse for all the space-wasters in my studio storage area to exist.