Nobody's scared of Sync because it's closed-source. People like open source software for dozens of reasons, including auditing, but this really isn't the main reason people are asking for an open-source Sync app.
For me and many other FOSS advocates, it's just a nice thing to have as a power-user. See something broken? No problem -- fix it and send it. The project gets free labor and the user gets a better experience. Many projects retain absolute control over their UX while still keeping the source open for optimizations & fixes -- it's not an issue of creative control.
There's exactly one good reason for broadly used software to be closed source: to make money. It's simply harder to sell an app if people can compile & sideload it for free. FWIW: I think that reason alone is already a perfectly good justification for ljdawson keeping the app closed-source if that's what he chooses to do.