I would expect that anyone working in this business to know why it is dangerous, simply because its part of their job as user. Furthermore, without loading and reading each label, how will your library/application be able to quickly identify each cassette? The label is the first step in identifying quickly any cassette, once loaded, some, but not all applications then compate the magnetic label again....a full audit of unlabelled tapes can take some hours, depending upon the druive type and the number of cartridges. The same using labels takes seconds......!
I am involved almost weekly with customer problems concerning wrong or missing labels, the hours of extra work are quickly offset with a good set of labels. If you are concerned with somebody stealing your data, they can still do it with or without labels! Your security should be better than that.......