Uncommon does not mean it never happens. I for one like to group all my unit-producing structures together, and sometimes I select both units and structures simultaneously.
A very good point!
It's intentional, yes: resources can't move, form formations, build structures, train units, research technologies, etc., therefore they don't have the right and left selection panels. In contrast, (many) units can, therefore they have those panels, even though they're occassionally empty.
This is not trivial. Most of the GUI is defined in XML or JavaScript, however, the minimap is partially generated by th C++ source code, complicating things.
I believe the general idea behind 0 A.D.'s current session GUI design is not to swamp players with too many numbers. Whether an unit is an archer or javelineer is more important information than whether e.g. their attack damage is 6.9 or 7.1.
That shouldn't be too difficult to implement. You could have a look at the gatherer counts patch (D3155/rP24357). I don't know if someone is already working on an idle worker counter( @Freagarach?).
A lot more is done than what's advertised on the forums. I recommend playing the svn development version from time to time.