While it may add somewhat to the adventure, I see two immediate problems with night time cache hunting. First and foremost would be safety. All to often you read in a Find Report something like "thanks, I'd have never know this spot existed if it weren't for your cache placement". That is a good indication that people are often going into places they are not familiar with. Doing this at night when visibility is greatly reduced (even with the best Brinkman) may not be the smartest thing to do. Especially when you consider that few, if any of these caches, are placed with night time hunting in mind. Definately an increased risk of injury, or just plain getting lost.
You also need to consider that a good number of your shall we say, "more dangerous" wild animals are noctournal. That means you are more likely to bump into that mountain lion in the midwest, or panther in Florida at night. While it would certainly spice up your Find Report, I am not sure the few dollars in trinkets would be worth the persoanl risk.
And finally, one of the biggest opponents of the sport are those who think Geocaching will lead to the trashing of our parks. Well, let it become widespread at night and they would probably be right. I am certain that the reduced visibility in darkness would greatly increase the chances for trail damage. That delicate plant life you easily see, and gently step over or around in the daylight is most certain to be missed and trampled by moonlight. :sad:
Just my thoughts but, I think night time cache hunting would be a bad idea. I will reserve (and recommend) it remain a daytime sport.