Why is it necessary to have both cone and rod cells in the eye?
First off - both the rod and cone system measure light intensity; both sensory cells increase their responses when excited by more photons (Purves et al., 2001). Hence, your introductory sentence that rod detect luminescence and cones color is incorrect. They are both sensitive to luminescence. 

You are correct that rods detect gray scales, while cones perform color coding. Both systems can detect gray shades; while rods are limited to that, cones are not. Rods are very sensitive to light and in a dark-adapted state are capable of detecting a single photon. Hence, they operate under conditions of low-lighting at night and as such they mediate night (scotopic) vision. Cones are photon-hungry and need more light to operate (photopic conditions).


Why have cones? Because color vision helps to detect and differentiate stuff.
Why have rods? Because night vision is helpful.
Why have both? To be successfully equipped to thrive in both light and dark environments.
Do we need them both? No - even nocturnal hunters that don't have rods have been shown to be able to hunt at night. Not only that, these geckos have been shown to be able to detect color in starlight (Röll, 2000). Under these lighting conditions, human retinae rely on the rod system and are unable to detect color. 


References
- Purves et al., Neuroscience, 2nd ed. Sunderland (MA): Sinauer Associates
- Röll, J Neurocytol (2000), 29(7): 471-84