Consider the following example
This will give an error even though expl3 loads etex for allocation to
the extended pool. Inserts to have a set of registers available with the
same numbers, and these need to be in the standard pool.
The issue isn't unique to expl3, and doesn't show up unless you add
quite a lot more register use on top of it. However, it is there. For
expl3 I wonder about using the extended pool from the word go
which will make it much less likely to happen. However, even then the
issue would remain for non-expl3 packages which expect etex to deal with
the register limitations of LaTeX2e.
How do people see this in general. One obvious approach would be to
alter \newcount, etc., so that when etex is loaded everything uses the
extended pool except for inserts. However, that might be overkill!