I'd say that an MMO needs a fresh infusion of major content on a yearly basis to keep players interested.
That doesn't necessarily mean a new expansion, especially for a brand new game. But stuff like new events, new major quest storylines, unlocking previously locked areas. New goals.
The mini expansions were not only an experiment in content delivery, they were about all the XI team could scrape up since all resources in the company were being poured into XIV and then ARR. ACP was the only one I didn't really like out of the six. Abyssea was a series of "battle" expansions officially, but even it had a badass FF worthy storyline behind it.