I frequently argue points I do not personally hold. This sometimes gets misinterpreted as me holding the opinion, or an attack on the person, or some such. But really I am testing the idea and recognizing that someone else could hold this opinion but not be in the room to advocate for it.
Unless someone prefaces their statements with a disclaimer (like "just for the sake of the argument"), I will assume in good faith they believe their own statements. Any later claim to the contrary leaves me with the following choice to make: were they lying then, or are they lying now.