Well, 1/10" = 2.54 mm is one standard. There are others and have been many. It is much a matter of what is readily available and reasonably compatible.
I certainly was not suggesting any other connector type, but saying the placement is somewhat arbitrary.
In which case there is an argument for using them for "power out" as well. 
You can end up with a "screwshield" form.
Depends on just what you are designing.
The UNO form is intended to be used with a shield. It is not appropriate on its own for connecting more than one or two attachments. The Nano/ Pro Mini can work with a breadboard. If you need to actually construct a usable device, you either make a shield for the UNO, or mount your Nano on stripboard or a custom PCB. Or perhaps you use a Nano with an "expansion shield" which multiplies the ground and supply conenctions.
