An actor friend of mine questioned the whole idea of including lip sync within act4ward.
Lip sync has by association a poor reputation through the many awful examples on You Tube.
However as an early fun technique with a new performer to the work or as practice in direction and control, movement and precision it is really useful.
Also the technique is used quite a lot in drama in television and ofcourse most location filming has an ADR soundtrack - which an actor has to provide after the filming. A few lip syncs can certainly help with thinking as regards marrying performance on film with a new soundtrack. If nothing else lipsync can break tensions during work and be a lot of fun too.