Nothing to do with software freedom, everything to do with security/auditability.

Yeah, with Dropbox software running on your machine, you not only have to trust them not to snoop on you, you have to trust their non-auditable code to be ~perfect~ against exploitation by others.

Unless you actually read through and understand your open source alternative line by line you aren't really running anything safer

Of course there is the argument that since it's open source it's safe since someone has "audited" it, but many times that's not true.

And even then unless you spend a lot of time trying to break it so you understand it completely you are way better off just writing your own solution, but that takes time and effort