If it can possibly fit on a lathe, we will turn it! The popularity is INVERSELY proportional to the cost for most of us, but the big money folks will buy it irrespective of cost. Do you have some?
I enjoy all kinds of walnut. Some here will argue that it is not suitable for pens. I disagree. I've only turned a little english walnut, but it turns nice. If you can get a mixture of Sap and Heart wood, it can be very nice.
For larger work and flat work, many consider walnut the king of woods. But, for pens, the grain is somewhat open and is difficult to finish. Plus, most is figure free and devotees of penturning like figure. Several have tried selling walnut blanks here without success. I tried giving away walnut blanks with no takers. Most recently, I posted an inquiry as to who might be interested in buying walnut blanks. Not a single positive response. Draw yer own conclusions.