I think they should bring back a DLC playlist. It is important for achievement hunters. Even though the DLCs are for free I believe not everyone playing has downloaded and installed them.
I have to admit that I haven't seen a DLC map in online matchmaking for a very long time.
The DLC playlist ensures that everyone playing that list definately has all DLCs. In every other playlist you will be mixed together with people who haven't got the DLCs, which is kinda a lot the case and as a result makes it nearly impossible to play in a DLC map.