a)Its needs translation per language and b)it takes longer to parse/process.
The weather information in iOS7's "Today" notification is an example (http://applenapps.com/wp-content/uploads/2013/06/ios_7_6_not...). It takes much longer to read the exact weather information than just seeing a weather icon and getting it (IMHO).
In this article's case, it seems he use something that wasn't "iconic" as an icon.