The question should be, "why use an optic instead of a reflector?"
A reflector gives a much better beam than an optic does. I wouldn't say that an optic puts out a broader hot spot than an reflector does either. I've spent a lot of time over the past year replacing optics with reflectors.
If you replace an NX05 optic with a 17mm reflector, the beam will actually be comparable, but the beam from the reflector is much more satisfying. I also believe that the reflector is a bit more efficient. Centering issues aside, the reflector gives a much more conventional beam pattern.
The statement that a reflector puts out a "small" hotspot is totally untrue, as a general statement. The smaller reflectors put out a nice sized hot spot for close to medium range use. The larger reflectors do have a smaller hotspot, but that is because you beam is more concentrated; however, this does give you more throw.
Flashlights are tools and like any tool, you have to choose the right tool for the job. If you are looking for something that will give a utilitarian beam like an NX05, but just better and cleaner, a 17mm or 20mm reflector is what you are looking for. The 17mm IMS reflectors have been a, pretty much, drop in replacement for the NX05 optics.