>>>>> "H" == Hans Aberg <[log in to unmask]> writes:

    H> Are there any reasons for LaTeX having multiple AUX
    H> files?

They are only necessary in an \includeonly scenario.  Some system
state (counters only at this point) from the UNincluded files is
stored in the aux files.  Otherwise they would be overwritten.

Unless this information were recognized in the sole aux file and
resaved when the part is not included.  I tried coding this.  It wasn't
hard to get almost right.  If you abort LaTeX in the middle of an
included file, you are in trouble because you are left with an
unclosed group in your aux file.

See my accompanying post and pointer to the code and documentation of
my partial solution.