Why are parameters to Bash's builtin optional?

[Victor Engmark]

(Re-posted from Stack Overflow 
<http://unix.stackexchange.com/questions/36751/why-are-parameters-to-bashs-builtin-optional>)

Running simply `builtin` prints nothing and returns exit code 0. This is in 
accordance with `help builtin`, which shows all parameters as optional. But why 
isn't this no-op an error? Is there a use case for this? A more useful result 
would be an error code or, even better, listing the currently available 
builtins.

Cheers,
V