The expansions aren't "worth getting", they're more or less required. WotG wouldn't be necessary if certain high level content didn't depend on it (and some would argue that content isn't important anymore, but I would say some of it is still valuable for progressing towards Audolin content). Abyssea is a must for leveling and empyrean weapons/armor, and obviously all new content in the current game requires the newest expansion.

The good news is you can just buy the ultimate collection to get everything you don't have for 1 low price.