Hi Danny, vertical tabs was an idea that I considered very early on in MoI's development instead of the current tab system.
As you know, MoI actually does already has a tab system that re-uses areas of the side pane, these are tabs like "Draw curve", "Draw solid", "Edit", "View", "Construct", etc....
Those allow switching sections of the side pane into different palettes.
There are some down sides to switching though - if you need to frequently use stuff between 2 different tabs then that takes extra time and effort to switch the area back and forth between the different modes. The worst case is if you need to repeatedly ping-pong back and forth between 2 things on different tabs.
By having tabbed areas split up into some smaller chunks rather than as one really big thing, it can help to avoid this problem because it is possible to have more sections open at once, like for example in MoI you can have "Draw curve", "Edit", and "Construct" all open at the same time. Then if you need to go back and forth between these things there is no penalty at all, so that is an extremely significant gain.
Believe me, it is very tempting to re-use space, and a lot of MoI's current UI design is based on space re-use. For example having the xyz control work as a readout and input both, having tabbed palettes, and having the command options area do double duty as an object properties panel in selection mode and then provide options for commands while running a command.
But unfortunately too much really broad space re-use can just lead to too much switching. It would not be bad if it switched into some major different mode and you were planning on staying there for a while. But many of the tasks that you would want to use the scene browser for such as selecting objects does not fit into that kind of thing.
The other problem with vertical tabs that kept me away from them is that the text on them is not as naturally browsable or readable as normally oriented text. It gives a kind of jarring feeling looking over it. It is something that you can get used to over time, but I generally like to try and stay away from things that have a somewhat strange feeling like that.
If I wanted to do switch the entire side pane to a different mode, I would probably use normally oriented tabs at the bottom of the side pane, like where I have currently placed the button to open or close the browser:
But I think the browser is a tool that will be used frequently while modeling, so that makes it hard to have a "modeling" and "browser" mutually exclusive modes I think...
Anyway, thanks for the feedback!
- Michael