In reply to The importance of typeface specimens for designers throughout History:
Because specimens generally accord equal status to all a foundry’s products.
And because extant specimens do not represent all foundries.
The types that are most culturally significant, being most representative of a time and place, i.e. those which were most popular, do not necessarily correspond to what one finds in the specimens in libraries.
It’s much harder to write a type history from commercially produced documents (trade set), because it requires much forensic expertise and effort in identifying the types.
As an aside, here is the earliest trade use of a sans of which I am aware:
http://pinterest.com/pin/49539664621179232/
Compare with the best-known first sans, the 1816 Caslon, which has never been seen “in the wild”.