Secret Code is Undesirable
"Secret" and proprietary is not the same thing, but the concepts overlap. For instance, proprietary code can be shared with "partners" and thus not be secret. However, rights (or contrariwise restrictions) associated with it are defined in such a way that typically only one entity is allowed to modify it, control its distribution and so on.
Secret code tends to be not only proprietary but also the worst type of proprietary. Secrets are kept due to fear the code is not secure (and seeing it would expose that), not tidy (an embarrassing mess), or possibly infringing (e.g. a GPL violation).
If someone wants you to use proprietary software, say no. Secret code is even worse. You might say, is this code so bad that you just hide it? Must you hide it?
Secrecy in governments and corporations is the same. Secrets are kept by people who don't wish to be publicly scrutinised; that often does - but not necessarily always - imply they're corrupt. █