As you can see, comparing the pattern with the following illustration (courtesy of Wikimedia), the term "guimpe" really is appropriate.
Courtesy of Wikimedia
While the McCall's pattern has the guimpe buttoned to the dress, more typically it seems to simply have been tucked into the dress.
A guimpe can also be a short blouse or a dickey worn under a dress or jumper. And from this image, there seems to be no difference between the guimpe and a "modern" dickey or short blouse:
The advantage of the guimpe is the advantage of any detachable insert, that is, flexiblity! Like the detachable collar, you could switch out one insert for another and create a different look.
And this is my little excursion into fashion history. :)