Imagine because of tradition that your team's preferred development method involved several people with a shared login, editing files on a build server using vim.

[Note that there are well known issues to do with only one person being able to edit a file at once, people going away from their desk and leaving the file locked in vim, system builds/restarts requiring everybody to stop debugging while this occurs. This is not what the question is about]

If source control was to be introduced without changing the workflow, would there be much benefit?

I am guessing that the commit history won't be much use as it will contain all changes by everybody in big lumps. So it wouldn't really be possible to rewind individual changes apart from at a really big level.