One possibility (that I've run into myself) is $corporate mandates that say that data can't be edited by hand, and that you have to write a program (to be executed only by those with sufficient authority) to modify said data.
Of course, another possibility is that this is more than a one-time thing -- maybe these book list files show up daily.
A warning about your one-liner (cool though it is), XML documents don't necessarily have to have newlines between elements. I've encountered some with no newlines at all -- I believe your solution would empty the file in that case.
Just some random thoughts...
Update: And BrowserUk phrases it better than I :)
In reply to Re^4: XML::Simple XML / XMLin / XMLout? or something else?