Active D lighting is doing something that you can reproduce in post-processing. It underexposes by about a stop and changes the tonal curve to lighten the mid-values. The overall result is that you have preserved the highlights and brought extra light into shadow areas in contrasty conditions. I quite often use the D-lighting option in NX2 to make a contrasty, backlit photo look the way it was to my eyes.

This is fine as long as:

1. You are shooting in RAW (you can do this sort of thing on JPG's but it quickly looks unnatural)
2. You have a decent RAW editor
3. You understood what I said in the first paragraph

If not - save up for the D90.