> I'd say it's a lossy way to express information.

A simplified explanation of the system is by definition lossy. This equally applies to a plain English description.

I’ve been in many design reviews and similar forums where someone has attempted to present a design through written English and finally someone says “we need a diagram here; this is too much to follow” and everyone in the audience nods because they are all lost.

One of the problems with trying to communicate system design with prose is that it makes sense to the person who writes it and has full context, but the audience is often left confused. Diagrams are often easier to follow specifically because they look under specified when they are.

> finally someone says “we need a diagram here; this is too much to follow” and everyone in the audience nods because they are all lost.

Yes, that happens. I can't remember any occasions where the diagram actually cleared things up though.

Coming to think of it, one way that seems to be pretty effective at getting complex designs across is in an interactive presentation with the presenter drawing on a whiteboard, starting simple and adding stuff while explaining what and why. The narrative is very important though. The whiteboard drawings by themselves are absolutely useless.