A recent discussion on the LaTeX Community forums has raised the point that
siunitx currently defines a lot of short unit macros outside of the scope of the
\si macros. I did this to match the functionality of the
unitsdef package, but I’m thinking of revising this for version 2. There will be an option to get
unitsdef-like behaviour (which I don’t think is actually very good).
Looking further ahead, I suspect I’ll be more definite about things when I write a LaTeX3 version of
siunitx. There, backward-compatibility is not an issue, so I’ll be free to do what is most sensible for the long term.