Only if you plan to level up till 85. Even if you only have the Main game, you will still get to experience the changes that Cataclysm brings, cos Cataclysm changes all the low-level zones as well adding new high-level stuff. But main game only allows you to level to level 60. Gotta get The Burning Crusade and Wrath of the Lich King and Cataclysm to level to 85.