Night sights allow a shooter to more quickly and accurately sight (front sight being the most important) a gun in very low to no light conditions. We are all required to properly identify the potential threat and what is behind it, so a quality tactical flashlight is probably more important. For night sights to be optimal, many find darkening the rears with a black permanent marker or contrasting rears further improves their use.
I would highly reccomend taking a night shooting course that incorporates the proper use of light and sight options. Too many people spend money on stuff, not training; it should be the reverse. I've seen many great range shooters and hunters humbled during a night shooting course.
Most self defense encounters are probably not going require any sighting, just point shooting, so one could argue the need for any sights.
Most shootings occur at night, often during the hours when most are at home and in bed, any advantage you can give yourself makes sense.
My bias is in favor of night training (you can simulate some of this with home dry fire drills) good tatical light and night sights.
Before laying down any hard earned money, try to take a night shooting course where you can explore what might be best for you - proper small tatical flashlight, night sights, laser, red-dot, or none of these. Proper defense ammunition with a low muzzle flash is another importan piece of the puzzle.
You might find "Night Master, "low light" shooting & flashlight techniques" by Bill Wilson and Ken Hackanthorn, albeit a little dated, very worthwhile. It takes a novice or above shooter, through the various night challenges and solutions. I'd spend money on this before anything but an actual night course.
Link provided for the DVD; or web search with many results
http://gunvideo.com/pgroup_descrip/63/4972/