The reason, which I know at least one person mentioned, is that the core game does not have Elite Specs. Elite Specs were introduced with HoT. It's a design philosophy more than anything as an evolution of the game, the story, and the player. However, the difference an Elite Spec and the core specs is really that an Elite Spec unlocks the ability to use a dedicated weapon and skill type. Elite Specs are by nature supposed to be more powerful. Yes, it creates a separation between Core players and Expansion players, but that can be said for all the other expansion features like gliding, mounts, etcetera too.
While it may not agreeable, it is the business model of the game and those features are designed to attract players to purchase the expansions. If we were paying subs, this would be an entirely different conversation.