There's no conflict between reading and understanding a codebase and seeing the value in a diagram. You can have both. Take a look at the example you chose: [0] and [1] (about a third of the way down).

They first explained PageRank (without a diagram), then explained the flow of the data from the crawler (with a diagram). You seem to be arguing that diagrams are never useful, and that just isn't true.

[0] http://infolab.stanford.edu/~backrub/google.html [1] http://infolab.stanford.edu/~backrub/over.gif