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 and \si macros. I did this to match the functionality of the unitsdef package, but I’m thinking of revising this for version 2. My plan is to change the default behaviour so that these macros will only work inside
\si (plus the
s column). 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-compatiblity is not an issue, so I’ll be free to do what is most sensible for the long term.