The primary difference is in the contacts.
Definite purpose contactors have general duty contacts. They are rated for motors and resistance loads. If they carry a rating for lighting it is propably only for incandescent types.
Lighting contactors contacts are specifically designed and tested for the inrush currents associated with Tungsten, Fluorescent, and HID ballasts and lamps. Also lighting contactors are not always "latching" types.
General use contactors are rated for typical inrush currents of 6 times full load, lighting contacts are often designed as high as 12 times. The lighting inrush also tends to last longer than general inrush currents.
While the NEC requires switches (and breakers used as switches) to be listed for the type of lighting they control, there is no such requirement for contactors.
So in answer, yes you could use non-lighting contacts if they can be sized correctly for the inrush of the load, but in the long run why bother.
