I'm always interested in seeing how coaches use the info available to determine how to coach their teams.  

I like the way David is running his L/MB combo.  

Liberos CAN serve and it DOES help a little when they serve and will get a better dig chance on their serve (DEF rating).

However, sometimes coaching serving (SRV) for both L and MB can result in some resources not being used.  Like why coach one of the 2 MBs that are Serving AND the Libero as only 2 of the 3 can serve in a match.

It looks like David is focusing on MBs for serving CPs and then not working the Liberos for Serving to use CPs elsewhere.