This does mean that you risk the chance of not playing the new maps very often until enough players with the new maps installed have entered the matchmaking pool.
If you have a party of friends who all have the new maps, have the “Matchmaking” option selected to “DLC” and THEN a new friend joins your party without the new maps, the system will recognize this and only place your party in matches playing the original on-disk maps to ensure that your party does not have to break up.