“I didn't have time to write a short letter, so I wrote a long one instead"
Interesting idea, but in reality you need to have many tools in your quiver to be able to make your point as applicable depending on boss' style, organizations accepted methods, type of meeting, type of content, etc.
Good communicators need to be able to use the elevator pitch, a short memo, an informational PowerPoint, a decisional PowerPoint, a speech, a succinct email, a detailed email, RFAs, action items, status meetings, working meetings, executive meetings, and formal reports as the situation dictates. Many situations will require several of these tools.
On the note of effective spoken communications I recently had the pleasure of taking Dr. Pfau's speaking course. He works with many organizations and if you get the chance to take his course I HIGHLY recommend it.