The theory is that traditional (incandescent) light bulbs waste lots of energy by producing more heat than light. Let's look at the bigger picture. I'm a big fan of saving energy: I go shopping with my car just once a month, I keep the thermostat at 50 degrees, I turn off lights when I leave the room. By far my biggest energy expense here in Massachusetts is heating my house. Ideally, I'd like to heat just the part of the house where I am. I accomplish this partly with space heaters, but it's hard to have them everywhere. It would be nice if there were little heat sources all around the house that would come on when I go there -- and there are! They're called incandescent light bulbs. They help keep my big heating bill down from October through May.
The only bad effect is in the summer, if it's hot, and I'm home, and I have lights on. I don't often have them on during the daylight hours, which last late in the summer. And even if it's dark and I'm home, I don't have them on if I'm watching TV. But once in a while I'm home in the evening and I'm reading or working and I need to have a light on. This is a relatively small consideration. And even this I can address, by having a few lamps around with energy-efficient, fluorescent light bulbs.
Here's a much better way to save energy. When you take a bath or a shower in the wintertime, don't let the hot water go down the drain. Keep it in the tub for a couple of hours to warm up the house.