Why should \lst@definelanguage be preferred over \lstdefinelanguage in a style file?

Question

The listings manual (v1.5b, subsection 4.18) recommends using \lst@definelanguage instead of \lstdefinelanguage in a style file:

Where should I put my language definition? If you need the language for one particular document, put it into the preamble of that document. Otherwise create the local file lstlang0.sty or add the definition to that file, but use \lst@definelanguage instead of \lstdefinelanguage. [...]

However, I can't find a reason for this distinction between \lst@definelanguage and \lstdefinelanguage in either the manual or the developer guide. Is the distinction important?

You may be wondering why I'm asking this question... Well, I've managed to get everything working in my style file using \lstdefinelanguage. That said, I'd like to follow the guidelines laid out in the listings documentation as closely as possible.

However, if I simply substitute \lst@definelanguage for \lstdefinelanguage, I run into problems. Here is an MWE:

\documentclass{article}

\usepackage{filecontents}

% --- generate the style file ---
\begin{filecontents*}{mystyle.sty}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mystyle}

\DeclareOption*{\OptionNotUsed}  
\ProcessOptions\relax

\RequirePackage{listings}

\def\mylanguage{Mylanguage}
\expandafter\expandafter\expandafter\lstdefinelanguage%
\expandafter{\mylanguage}{keywords={foo}}

\endinput
\end{filecontents*}
% --- end of style file ---

\usepackage{mystyle}

\begin{document}
\begin{lstlisting}[language=Mylanguage]
foo bar baz
\end{lstlisting}

\end{document}

Everything is hunky-dory with the code above, but if I simply replace \lstdefinelanguage by \lst@definelanguage, the listings packages throws two errors:

Couldn't load requested language.

and

language mylanguage undefined.

This seems to contradict the manual's claim that \lstdefinelanguage and \lst@definelanguage have the same syntax; at least, based on my simple test, they don't seem equivalent when in comes to expansion of their first argument.

Should I use \lst@definelanguage instead of \lstdefinelanguage and how can I get the former to work in my MWE?