I’m not arguing that Instagram should allow portrait images. I’m just saying the world is more interesting than just squares.
There’s a lot that is great about the square crop, but it’s also not a very useable image crop. You rarely see frames for square crops, and your displays are rectangular. In my house I can often be heard, rather rudely, barking at people: “why don’t you shoot that picture/video in an useable orientation.” Meaning: stop shooting video in portrait and stop taking group photos in portrait. I try to shoot 95% of all my photos in landscape, with only an odd few in portrait.
I simply find that every time I take a good shot in portrait, there isn’t much I can do with that shot that I actually want to do with it. It’s doubly annoying for square images. They can look great, but then what the hell do I do with a square image? It looks great on my iPhone screen, but outside of that it just looks like: where did the rest of the picture go?