Presumably this would be to allow sorting of bugs by last-modified
date instead of by creation date.  With the xml output, this is no
longer needed.  For example, I view bugs in mutt with
  $ be list | xml/be-xml-to-mbox | xml/catmutt
and use mutt to sort the threads by last-modified, e.g. by adding
  set sort=threads
  set sort_aux=last-date
to my ~/.muttrc.

That being said, I could go for a user-specified sort command in
becommands/list.py, rather than the current bug.cmp_full, since other
mail readers may suck more than mutt ;), and even mutt might not have
that perfect sort you desire coded into it :p.  The problem is that
while the cmp_* functions in bug are short, they are not really the
sort of thing you'd want to type in on the command line.  Perhaps we
can just slowly accumulate a rich array of bug.cmp_* functions as
they are requested, and allow the user to prepend their favorites to
the default cmp_full list...