Is it acceptable to copy verbatim the description of a software package? I just read an article where the description of a software package was lifted verbatim from the package's site itself.

It's a whole paragraph where not a single comma was changed. There are no quotations, nothing to indicate that this is a verbatim transcription of text. The site of the package is properly cited though, nothing is being hidden.

I understand that the description of a package created by the author is the best source to understand what that package does, but copy/pasting about 12 lines of text (single column) seems a bit excessive.

To what extent is this acceptable?
As long as a proper reference and quotation indications are provided, there is nothing wrong with it - as with any other type of verbatim text copy. However; it is debatable whether this is a desirable approach, and this may vary with the traditions in individual fields.

If no reference is given it is (a mild form of) plagiarism.