Of course they can stop being nice at any moment and start doing nefarious things. But having the client open source means that when this happens, you can stop using it without data leaks, and until that happens, you can also be sure about the security of your data and exchanges. As a plus, you can run your own server for you and a group of friends/collaborators/whatever, if you wish. In my eyes that's a vastly better alternative than (I would say most, but it would be inaccurate) all the non-decentralized non-federative alternatives. Plus, their whole mission being secure messaging (as opposed to a nice-to-have side feature) will probably make it harder to do a full turn soon, I guess. Even if Signal is on to something eventually, I believe it does no harm to take full advantage of it while we can, as long as we are aware of a potential turn of events.
