Well, as a general rule, I'm usually for exposing as much information as possible.
Now for other use cases, I could imagine some people wanting to personalize the letterbox scale mode either by having other colors instead of black bars, or even some special images/background/animation/ads
The scale outer could almost do the trick, but you need to know the original windows size set in the project if you want to emulate viewportRight/Bottom/Left/Top and Viewport Width/Height (here viewport means the playable area without the letterbox areas)
Also, in some games, like infinite scrollers, I handle the scaling myself to fit any screen ratio (using crop mode, unbounded scrolling and tweaking layoutscale and scrolling by hand). Having the original window size allows for a more consistant "game-space" through the different ratios.
Though, since I use unbounded scrolling, the size of the layout doesn't really matter anymore and I use those values instead.