Yes, they provide a useful service, for free. But you get sucked in, then you get locked in.
Matrix is Open (as in freedom, liberty) — you can move to another Matrix service provider and still talk with everyone, no matter which Matrix service provider they signed up with. Your work, university, or friends can host their own Matrix server, control their own data, modify it to talk to other systems, and customize it as they wish.
If you care about freedom of communication, use Matrix!