Which SWI-Prolog modules would you like to see ported to Logtalk (in order to be able to use them with any back-end Prolog compiler)?
I didn't express myself very precisely. It is not only for its modules that I usually write Swi-dependent code. The two main reason why I prefer Swi to Yap (and to other compilers) is that it produces good error messages (Yap e.g. fails silently when you try to use a non-existent predicate) and that most of its features are very well documented.
I would not be so bold as to post the following list in the wish-list part of this forum, but since you asked, here are my personal top 6 reasons for writing non-portable code in Logtalk or for using plain Prolog or Python instead of Logtalk.
(1) There are some things which one does not want to do in Prolog, but in C. As each Prolog compiler has a different foreign language interface, I do not use Logtalk for programs that use predicates written in C. I know that one can hardly expect the Logtalk compiler to take care of such low level things, but for me this is the most important reason not to use Logtalk for all my programs at the moment.
(2) There are some system libraries of Swi which have no equivalent in Logtalk. Sorted by importance: readutil.pl, sort.pl, ordsets.pl. (Because of the presence of readutil.pl, I have been using non-portable Logtalk instead of Python, lately.)
(3) Logtalk lacks some practical built-in predicates that are present in Swi. Sorted by importance: shell, time_file, exists_file (and the like), ground, format, atomic_list_concat, downcase_atom, atom_number, term_to_atom.
(4) Logtalk has no interface to databases. (I use sqlite3 and very ugly shell tricks.)
(5) Logtalk does not have sockets nor a web server (as Swi).
(6) Logtalk has no regular expressions. (If it had them, I would stop using Python).