You can duplicate an ultrawide by linearizing the fisheye shot in software, but you cannot do the reverse. Fisheyes have a diagonal field of view of 180 degrees - something physically impossible for a rectilinear (non-fisheye) lens. Also, after you convert a fisheye picture, the aspect ratio is different from the original, ie no longer 3:2; you'd need to crop some more.
i use a fisheye, as well as a Sigma 12-24 and the Canon 16-35. IMHO, the fisheye gives unique perspective that a normal wide angle cannot. i bought the fisheye on a whim, thinking i would sell it off soon; it has been with me for many years and has become one of my must-bring lenses for weddings.