It could be useful in a scenario where you trust the service provider to provide a legitimate encryption implementation. If I trust Eve to provide an E2E servi

It could be useful in a scenario where you trust the service provider to provide a legitimate encryption implementation. If I trust Eve to provide an E2E service, in the same way I trust Whatsapp (not a given by any means), then an open source implementation of those tools lowers barriers to entry. That's a mixed bag in that it potentially enables bad security models, but overall I think access to tools is a good thing