Faced vs. spine is a shelf thing; in rack pockets everything is faced. But there's a fairly standard size, and it turns out you don't want to get most of your authors above the length where fewer than "n" books fit in a pocket (I think n=3, but I'm less certain on the number than on the basic concept).

I'm still remembering sf/fantasy authors being *cut back* on acceptable lengths a while ago, after the initial growth through the 80s.