Beatrice Warde thought about typography in the way “What must it do?” and the answer is to be invisible to “(…) conveys thought, ideas, images from one mind to other minds”. I think this is the essential purpose of typography and sometimes a project may not need anything else than this, to convey. For example, in this case for the signage of the US, they chose Interstate just because it contributes legibility (example). But many other times you need to express with typography, to associate a project with a voice that different typographies let you (example)
Leif Steen
LCC MAGD GROUP V 
Is that an agreement or disagreement to Warde’s statement? As her text is mainly concerning book-typography, and this of course has to do a lot with readability/legibility (‘invisibility’).
On the other hand she’s also talking about the type’s FUNCTION, its purposiveness and how it is a ‘tool’ to transfer and receive thoughts and content.
I think her statement is still valid today — it just has to be thought in a broader way, as technology, culture…etc. and therefore the possibilities changed so much since 1932.
As you said, the text in mainly concerning book typography and, in this sense, I agree with B. Warde that type most be invisible to the reader.
But there is a point in the text where Warde talks about advertising: “And if what I have said is true of book printing, even of the most exquisite limited editions, it is fifty times more obvious in advertising”. Today’s advertising is not like 30’s. You don’t have those inmense texts because nobody is going to stop and spend 5 minutes reading your ad. Today you have to capture the attention of the guy walking through the street in 2 seconds and the important thing is to communicate a strong identity or an idea and typography have in many cases an important role in this. It’s not just about readability/legibility, it’s ALSO about the “voice/tone” that the typography conveys. From an elegance and serious ad of Mercedes http://www.emercedesbenz.com/Images/Jan06/31NewMercedesAdCampaign3SmallMain.jpg, a funny campaign of Nike http://www.ibelieveinadv.com/commons/nike_italy_zambrotta.jpg to the creative and young campaign of Ikea http://lh5.ggpht.com/_n7wvHC-Fuk8/Snoz4zLf14I/AAAAAAAAAs4/yJCsGQMiWco/Joy.jpg the importance of expressing through typography today is very important in advertising.
nice examples!
Could be that I give her too much credit 😉
But there are some key phrases, like her citation of someone saying: “…what must it do?” which I understand as being projectable (in a wider sense) to any today’s design-section – also advertising.
But yes, I totally see your point!