With an ultrawide zoom vs. a prime, particularly with a lens designed for APS-C (crop), the main advantages are versatility and availability. When you get to the very short focal lengths, a single millimeter on the focal length can change the composition significantly.
There is also the issue that depending on how you define "wide angle", you may not be able to find a prime for crop. Most of the wide and ultrawide primes that exist are for full frame, not crop. On a full frame camera the ultrawide-to-wide range in focal length is 14mm to 28mm. For crop, it's 9mm to 18mm. This is one arena where you can't reasonably expect a single lens to do the same duty on both formats, because of how the crop factor changes the field of view. With a D7000 as your body, your choice of ultrawides are mostly going to be zooms. I think the only choice of prime that truly is ultrawide on a crop body is Samyang's 10mm f/2.8 lens. And that is only cheap because it's an all-manual (non-CPU) lens. It doesn't report to the body of the camera, both focus and aperture are manual and not controlled by the camera body, and you'll have no lens EXIF information. Samyang's 14mm, while certainly ultrawide on a full frame, is only a 21mm equivalent on a crop--wide angle rather than ultrawide.
The main disadvantages of the zooms vs. the primes, are image quality compromises to accommodate the zoom range (although wide angles in general are more difficult to design without CA, vignetting, distortion, or corner softness issues than normal or telephoto lenses), and slower maximum apertures on the lens. Few of the ultrawide zooms are f/2.8, while some of the fastest wide angle primes for full frame are f/1.4.
You could also use a longer lens, like your 50mm, to shoot images you can stitch into panoramas and get higher resolution and possibly higher image quality, but the feel of the stitched panorama will not be the same as using an ultrawide lens.