A fair question......I've been racking my brains to come up with a downside though.....LOL!
Actually, although personally I totally LOVE the all-inclusive concept, there are those folks who don't, and their primary reason is that they really like to spend time exploring away from the resort and don't like to be 'tied' to having all their meals and libations at the resort, and don't want to pay a package price that includes meals if they'd rather eat in town while exploring.
While I love just sitting on the beach or by the pool with a good book, it's for those who don't that the resorts have responded by greatly increasing the dining options (so that you can literally eat at a different restaurant each night) and by offering easy access to booking off-site tours and activities......for some folks that's enough to satisfy their needs.
Another consideration is that while many all-inclusives are located on gorgeous beaches, only some are located close to a town or cultural center........so if you really love to spend your days exploring museums, historical sites, or shopping districts, and discovering small popular-with-the-locals bars and restaurants, then you'll have to weight how much of that you'll do versus how much just enjoying the beach and pool, and it could be that an all-inclusive isn't the right choice for everyone.
I hope this helps!