There are a thousand different ratios acceptable for posters, as long as the poster stays between 1:1.33 and 1:.1.67. That means a 600 x 800 image all the way up to a 600 x 1000 image and every ratio in between. I think that's a pretty decent compromise. There will always be some strange formats not supported.

There are about 20 different poster sizes/aspect ratios available worldwide, yet the site is ultimately limited to 4 portrait oriented sizes. I bet that's why many of the entries have dvd/vhs art instead of theatrical posters... but whatever, Idgaf anymore

You are speaking from the perspective that TMDb should be a poster archive site, which surely isn't anywhere close to what a regular user is expecting? As I'm sure you understand, featuring a poster in some extreme format is not very helpful (from a design perspective) for TMDb homepage users, third party sites or third party home users. I know there have been some suggestions of that kind in the past, including additional options for backdrops, but as of now I don't think it's a high priority for the site.