The way the question is framed, the only possible answer is "yes", because the base game (TS2, TS3, and TS4) is so terribly limited compared to whatever was in the previous iteration. Thus, it's "necessary" because the entire game is designed for it to be necessary. However, the proper answer is that it shouldn't be necessary. It should be cool, and expand opportunities in an already fleshed out game sufficiently that we want to purchase it. That's the way most DLC work in most (definitely not all) other games. DLC should never be necessary.