Portrait mode seems like. no brainer for reading PDFs and writing documents like scripts or email. I sometimes take notes with my pencil in either mode, but I only read notes I've taken in Noteshelf (2, 3) in portrait mode. The only time I really want it in landscape is if I've made a drawing or diagram that calls for landscape.
So, whereas many apps have great presentation in landscape (Apple News, Music, DaVinci, etc., it's not ideal for everything, and it's a shame the Magic Keyboard is so restrictive. At least Apple gave us a face time camera that doesn't make us look like we're watching TV or solitaire when in a conference call.