After reading the post about PC's trying to weasel out of using light sources on Planet Algol, I got to thinking about this. Usually, a mage just has continual cast on stones or his staff, so having a light doesn't really become an issue. My big thing is the radius of the light source.
Many light sources only have a 15 to 30 foot radius. If you're walking into a big cave that's 150 feet across, you'll stick out like a sore thumb to every monster there. If they have bows, you're kind of screwed. You'll just be walking along, and arrows will start pelting into your group. You'll see the arrows appear, but you won't have any idea where they are coming from. I think many DM's miss this.
The players will need to either use magic to locate enemies, have a demihuman locate the attackers, or launch torches or lighted items out around them, or they will have no idea where the attackers are located. Most dungeon adversaries can see in the dark just fine, so they don't need light sources. Because the players have torches, every monster anywhere nearby will see them coming, and will have fantastic targets.
I'd give monsters total cover until they have been located. In addition, they might even gain a bonus to hit the torchbearers, since they may as well have a giant bullseye painted on themselves.