I think that on any given night, they have a 'quorum of musicians' so someone else can jump in pretty quick if one doesn't know the song. Also, these cats hang out at COGIC conventions and traditional COGIC churches, so I'm pretty sure they are fluent in all of the old bishops' favs, lol. I know I am getting pretty good at all of my Bishop's hymns, if he'd quit changing the key every time he sings them, lol.
I have a friend, Gabe, who is a nat'l musician for COGIC, and he knows way more old stuff than current/new stuff. I think there are still some pretty traditional COGIC churches that sing the hymns, call and response, and have the hand clapping foot stomping sound over the contemporary. Those churches breed the types of musicians that play at the convocation.
To answer your question, I think adequate varies depending on what you want to do. If you will only serve a particular church or style of church, then adequate is having a grasp of their repertoire of music. However, I think a good musician can easily adapt and become adequate at another church or style... and a lot of stuff crosses over with a few minor differences.
