When I have free time at safety conventions I like to walk around and peek in break-out sessions as long as I can do it without causing any type of distraction. Most times, yes, most times, the rooms are dark.
Speaking in a well-lighted room adds energy to the experience while you are presenting. A darkened room adds focus to your slides but lulls your audience if the slides are not exciting. While you talk in a dark room, it hides your body movements which includes your most powerful communicating tool–your face.
There may be a few slides that need to be seen as clearly as possible. Just darken the room while you show and speak about them. Then turn the lights back up for the other parts of your talk. So everyone can see you—the main attraction!