Hello,
This is a *very* useful feature on Wikisource. It should make it easier for readers to understand why they can't find a popular modern text they were looking for, and help curb repeated recreation of copyrighted texts.
The deletion log has been publicly viewable for as long as I can remember (with a convenient link from the messages on inexistent pages), so it was always a bad idea to include personal attacks and other excerpts in the reason. This is a good reason to education administrators, not remove the feature.
If the English Wikipedia decides to disable the feature due to poor deletion summaries, please disable it on a wiki-by-wiki basis. I would hate to lose it on Wikisource.
Yours cordially, Jesse Martin (Pathoschild)