It is a bit of a continuation of the somewhat annoying trend of integrating features into apps that should be part of the window manager (tabs, in the first place, for example). This one is extra awkward because even windows (which has spent a lot of time behind on window management) can do two things side by side as the same unit now.

I just think it goes to show how little the window management of desktop OSes has improved over the years that desktop applications have had to up the ante...

I also think the differing behaviour between different apps implementing split panes (e.g. keyboard controls for creating/switching) is very annoying. Somtimes this flies in the face of any desktop's native window splitting or tab support as e.g. an app stops supporting multiple windows. For example, current browsers don't have a good way to configure usage without tabs, and at some point removed support for setting the window icon to the site's favicon.

Labeling a legitimate question as “negative” while not really answering it is unproductive.

Huh? There's actually more stuff in the way than if you used two windows since the divider is actually larger than the gap between windows is.

The only way this saves on space is if you're using vertical tabs.