Probably now more than ever before you can buy some of the most fantastic mill spun yarns you'd ever hope to see. Certainly you can get by without spinning your own yarn but, unless you own a mill that caters to your every whim, or have hired a personal spinster who twists fiber to you bidding you will always be limited to only what is available. If you develop the skills to spin what you want then the creative control you have over any fiber project increases 100%!
Owning a spinning wheel, and knowing how to use it is like owning a miniature yarn mill that caters to your every whim. (And, a spinning wheel is a great deal more affordable.) True, I've invested a good sum of money in hand spinning equipment and fiber. A person can buy a lot of yarn for the price of a quality wheel alone. The investment in fiber studio equipment is somewhat like setting up and supplying yourself with a personal kitchen if you enjoy gourmet dining. It would make no sense to have a good gas stove, a refrigerator, copper pots and pans etc. if you never ever eat or if you only enjoy fast food. I have a voracious appetite for high quality yarn. (Since I think of yarn as the food of my project work I'll continue with this metaphor).
If I know I want to eat gourmet food all the rest of my days then I need to either spend a great deal of money on fine dining or learn to cook well. In the long run dining out is going to cost me much more and my choices will forever be limited to what is on the menu and the hours of operation. I'll also have little control over portion size. (Ever buy a whole skein of yarn when you only needed a yard or two of a specific color? Did you get the exact color you wanted?) What if I'm in the mood for just one of a very unique Italian egg roll at 4:00 a.m.? If I couldn't make it myself I would constantly have to settle for something other than what I really want in my work.
By the way, while I admit I'm a yarn snob I am anything but a connoisseur of good food. Like most addicts I get so lost in my substance of choice (wool) that I forget to eat altogether. When hunger does intervene I grab whatever is edible, quick and easy. Sad really, but honest.