I'd say it's a lossy way to express information. I find that architecture diagrams often cannot express the exact concepts I mean to communicate, so you're left trying to shoehorn concepts into boxes that are somewhat similar, and try to make up for the difference using a couple of cryptic words.
Prose doesn't look as nice, but allows me to describe exactly what I want to say, on any level of detail required. Of course, like with a diagram, you do need to put in significant time and effort to make it comprehensible.
