Because there is the chance that the intruder isn’t really a "threat". Like I wrote earlier, there are lots of newspaper reports of homeowners shooting their relatives by mistake. Here’s a few:

Man shoots fiancée:
http://www.ktnv.com/global/story.asp?S=11296215

Husband shoots wife:
http://www.hcnonline.com/articles/2009/07/21/conroe_courier/news/shoot0722.txt

Cop shoots daughter:
http://www.masscops.com/f79/ct-officer-shoots-daughter-mistake-31261/

Man shoots wife:
http://www.chicagobreakingnews.com/2009/03/wife-shot-elgin-torrance-clark-reckless-discharge-firearm-burglar.html

Man shoots wife:
http://kathmanduk2.wordpress.com/2008/01/02/natchez-man-shoots-wife-in-mistaken-home-invasion/

Unless you live alone, that bump in the middle of the night might not be what you think it is. I’d rather strobe an intruder to determine it’s a threat, than shoot "someone" and later learn that I was my unarmed neighbor’s kid after my laptop, or my boy sneaking back into the house after being out late.

I’d suggest that before you decide not to have or use a light, try taking a low-light class and then make a decision.

Chuck