Why would you want to assess what's actually running on their servers? Even if you know the code on it, you have no guarantee it's not running inside a hypervisor that logs the contents of memory, or something.
In terms of cryptographic robustness, it's good for an app like Signal to have a closed-source server, because it forces you to not trust the server.
(This is of course separate from whether it's good for the Signal server to be free software for inherent free-software morality reasons.)
True, but it seems like wishful thinking for Internet users to reasonably assess the dangers of trusting the 'Cloud' with private data.
Still, I think the most important breaches of privacy are not necessarily in decoding the messages themselves, but rather everything from location data to contact lists including time and number of communications these apps have access to by default.
In terms of cryptographic robustness, it's good for an app like Signal to have a closed-source server, because it forces you to not trust the server.
(This is of course separate from whether it's good for the Signal server to be free software for inherent free-software morality reasons.)