Yeah I see what you mean, and an idealic situation it would be the same.
However, problems i've seen with creating mirror scripts for rigs before is the variation.
Not all riggers will call their controls ".left" or ".right", personally I prefer "L_" and "R_" as a prefix. BUT this can easily be overcome by getting the user to determine the naming convention and position of it.
With that problem taken care of, another I come across is the way people orient or mirror their controls. Some use -1 in scale (I personally dont like this as the mirrored object rotates the opposite to the manipulator), for FK controls I mirror the behaviour of the joints and orient the controls to the joints, the translations dont work well together (in local) but rotates lovely. IK I differently, as its mainly use for translates instead, and our animators prefer using the same orientation for both IK sides.
So for the way I rig, Ive got a clear Idea of what attributes can be directly copied over and what attributes needs to be reversed, in order to make a successful mirror/flip script.
When I get a spare moment i'll have a think and write one up, and just see what I can do to make it as generic as possible.
(i'm hoping other people might bring some more info to light as to whether im getting the wrong end of the stick here or whether i'm giving crap advice lol)