I don’t wish to be drawn into a long running debate so I’ll keep this as brief as possible.
While there is absolutely nothing wrong with the way that @gex performed his pseudo-mirror operation (I’ve done this myself many times in the past) it is certainly not the most ideal or efficient way to perform a mirror operation.
While copy/move/rotate 180deg has it’s place, it’s only ideal in specific cases and actually requires the same, if not more actions to perform.
In the first example I had to perform the following (8) operations:
select sketch (by drawing a marquee over it)
select move
select copy
move cursor to axis point
switch to 3D view
select rotate
type 180deg
select top down view
In the second example I performed the following (6) operations:
select sketch (by drawing a marquee over it)
select Command+F
select Mirror
select axis point
select Confirm
select sketch mode
Notice that I did not lose the selection. This is because I never exited sketch mode and instead used Command+F to invoke the command tools while still in sketch mode.
Once the operation has been completed you are indeed kicked out of sketch mode but a simple click brings you back in.
You’ll notice that using the first example it is necessary to switch to 3D view in order to perform the rotation around the axis before returning to 2D view afterwards.
While many people like to sketch in 3D view (myself included) many people sketch exclusively in 2D view (architects, engineers etc) and switching to 3D view each time would quickly become frustrating.
In the third example it becomes clear that the Mirror command is invaluable and once accustomed to the shortcut it becomes second nature.
