No, you do not have to have circulars, but very helpful with a large number of stitches. I sometimes just use the 14" but the work gets so heavy it's murder on your wrists. Clover makes a "Flex" needle that has two, short, wooden needles with points and each needle has the plastic cord like a circular with a large bead at each end to keep the work from coming off. Those are great and very easy on the wrists.
I use circulars for hats, sweaters, bags, wrist warmers, and other tubular stuff that I don't want to have a seam or just to do circular, flat, radial pieces.