By removing the *.glade files in /data/ui/, the preferences window, the launcher plugin and the netstatus plugin are not translatable anymore, from what I can see.
They look like they can be translated because the .po files are the old ones and the translation template in Pottle is too, but if some new code is added that needs translation, when regenerating the translation template those entries will all be commented out.
Adding or not the *.ui files to POTFILES.in doesn't make any difference.

There's something else. In and older commit ( http://lxde.git.sourceforge.net/git/gitweb.cgi?p=lxde/lxpanel;a=commit;h=6ab0e0b8952991fb74a9d7954afdd50171cc190e ) a couple of strings in the launcher plugin were made non-translatable. This may have been done on purpose but it may have been done accidentally too and the .glade file is needed if it's to be fixed.