Good question.
A previous discussion that may be of interest:
http://www.elkarte.net/community/index.php?topic=210.msg1428#msg1428
I would phase it out honestly.
Yes, certain addons will ad a single string and an entire file just for a string is overkill, but a file just for nothing is a bit overkill as well.
Additionally, if in some next version we would start "scope" language strings, and use a class or something instead of the $txt directly, autoloading would be easier with a file for each addon.
For example:
addon.{addon-name}.{file-id}.string
would be enough to instruct the "loader" of require the file /ADDONSDIR/addon-name/languages/file-id.{language}.php without even have to explicitly loadLanguage('file-id').
emanuele dreams