The rotate left/right and flip horizontal/vertical buttons are so close to each other on toolbar but behave different on multiple selected objects.
When selected multiple objects and rotating, all objects rotate around their centre. To have them rotate as one, you first have to group them, rotate, ungroup.
When flipping multiple selected objects, they already flip as group and see no option to flip these by themselves.
I think these should behave similar. Most of other programs treat selection as group, and this is what I first expected of Inkscape. But considering the fact that grouping and rotating is quite easy, so having these actions both per object would make these actions more general and therefore more useful.
And to mention, I would only file a bug, no actual coding yet, Inkscape codebase is a lot like Chinese for me (refering to something ultimately complex).
Also, thanks for this good program Mattias