Light weight models are created to make the graphic computations more manageable. It allows very complex assemblies to be loaded without all the overhead of parametric data. They essentially become bound surface representations (bound meaning they can be represented as filled solids)
You should be able to use them in drawings but they may not have the intelligence to create detailed annotation features from them. Since they are bound surface models, they can still do hidden line removals and such.
I don't know the specific extensions you refer to but they simply are part of the simplified data sets generated by Creo.