Joseph Wright writes:
 > A quick look at l3names and source3.pdf suggests there are a lot of
 > other primitives which only have :D names (particularly the pdfTeX
 > ones).  However, I suspect most of these should only be available within
 > the kernel, so it's probably not an issue.  (For example, things like
 > \skipdef are only :D, as far as I can see.)

yes something like \skipdef has no place anywhere other than in a single place
inside the kernel, where it is used to provide a new register in my opinion. I
mean either we abstract and provide an interface to "skips" etc or we don't. 
One of the problems with the TeX language is/was that all such levels got
horribly intermixed, which might have been ok for something like plain TeX,
but produced some level of chaos in any bigger format.

I wouldn't claim that our judgement on what we want to provide outside the
inner part of the kernel is flawless, but with a few exceptions I hope we got
it more or less right.

So right now while we are making the last set of major changes to the core
part of expl3 this is the time to point out any such mistakes or omissions,
please

frank