Does stumbling around in the dark hurt me?

Solution 1:

As a fan of the Series M&M Part 6+ i can hardly recall having any gameplay advantages using Lightsource Spells. Except the different vision all do the same : Brighten your enviroment.

Here is a forum post from the off. forums from ubisoft:

BTW, the game does not penalize you in any way for using a light spell of some kind--you are not "less visible" to your enemies...;) They will attack and fight the same whether you have a light spell going or you don't. Likewise, your attacks against your enemies are not degraded if you cannot see them, either. This condition basically means that you and your enemies have the same advantages in terms of dark-vision; neither has an advantage.

Source : Ubisoft Forum Post