OK, lets look at UA/AC ORD-YYZ.

Like you (UAEmployee) say, together, UA and AC provide roughly hourly service. But, isn't that because they are in an alliance and have decided to co-operate rather than because they codeshare?

I've looked up some examples in online booking engines, and it seems as if, on a roundtrip ORD-YYZ, I can use any combination of AC/UA aircraft and AC/UA code and still get the same fare and fare rules. So, what is the real benefit to me as a passenger of having the opportunity to book a flight under a code different from the operating carrier's?

If the fare or routing rules don't allow a mix of carriers there are at least two possibilities, one is to introduce codeshares, another is to rewrite the rules to make mixing the carriers "legal." One of these solutions is transparent to the customer, the other leads to unnecessary confusion. I still don't see why the airlines can't choose the solution which is more straightforward to the passenger.