It is true that to create all the colors on the color wheel an artist only really needs red/blue/yellow or magenta/cyan/yellow, depending on the color wheel preference. Some artists restrict their colors to only those three. The reasoning is that all the secondary and tertiary colors will be made up of the same primaries and will therefore be harmonious. That makes sense when the pigments are bound together in a paint.
The procion dyes are not bound together. When I mix a red and a blue together to make purple, I still have red powder and blue powder rather than purple powder. Each of those powders have their own unique properties meaning they will bind to the fabric (with soda ash) at different rates. This is what creates the "color splits" that we dyers talk about all the time. The 14 pure colors won't split. The 14 colors mixed together will split (give a different color aura) depending on each color's unique properties.
So, in low water immersion dyeing when I want a mottled fabric, I might choose to use more than one pure blue so that the blue dyes might split and add extra depth and interest to my finished fabric. Just think of the possible splits you can get by using one pure red and multiple pure blues when mixing a purple. I might even use the turquoise blue dye that has a bit of green in it in order to desaturate the red dye and add that additional element in the finished product; I might end up with a turquoise split. If I used a yellow to blend with the blue for a green, I might end up with a yellow split. It is all about the possible splits.
That is why I will use all 14 pure dyes from Prochem and, if I were in Europe, I would be using the additional pure colors they have access to. To better understand the properties of the pure colors is why I created my own dye color "wheel" and will be making a color dictionary.
Here is my color "wheel." I simply placed a few drops of each pure dye concentrate where I thought it would fall on the color wheel. I could have been much more restrained and tried to constrain the colors but I chose to let them bleed together. You will notice that I did go back in to circle and label where I thought the color was most true. And here are detail shots.