First of all, I don't play nightstalkers, so this post may be of little use to you. That said, I use individuals mainly for three things:
a) Inspiring (not applicable for NS)
b) Disordering. With their 360' Line of Sight and long charge distances (especially while mounted or with the wings of honeymaze) they can easily charge and disorder fliers threatening your flanks/rears; they can run over the board to disorder shooters and/or casters and often they can disorder cavalry taking their TC with them. For this purpose alone I often take one or two cheap mounted individuals.
c) Chaff. While individuals cannot block killer units completely (due to the overrun rule), this overrun is at most 6". So if your enemy unit in front of your infantry, hold them up for a turn with a individual character and move your infantry 7" behind your character to get the first charge.
Other than these are of course the caster dudes who have business casting spells.