A number of people, like Trailrunner, wear the insulated jacket and pants to "stretch" the temperature rating of a less-warm sleeping bag--for instance, to take a 30* sleeping bag down to the low 20's. It didn't work for me. Of course, I'm a cold sleeper!

As already mentioned, using the clothing without the sleeping bag exposes more surface to the cold outside air. Another problem is that clothing that lets you sleep comfortably at, say, 20* will be probably too warm for even the minimal moving around camp when you're awake. You'll get sweaty, which is something you want to avoid in cold weather.