I agree about wearing clothes in a bag, though I tend to select a bag to match the coldest expected conditions and use the clothes as a safety margin for unexpected drops in temperature. However, I've known others who successfully use them as an integral part of a sleeping system.

Regardless of whether you consider them an integral part of your sleep system or as a safety margin, there are two conditions of using clothes in a bag:

1. The clothes have to be dry. Wet clothes simply soak the bag and make you colder.

2. There has to be enough room in the bag that the insulation in the bag and the clothing isn't being compressed - if it is, it's not going to keep you warmer.