I've walked away from the B-A segment in the A-B-C-B-A scenario above. It has always been due to a change in my plans after I bought the ticket and ended up costing me money (buying a new B-A ticket to get home later). I think most of the tariff violation rules are written with an eye on people looking to circumvent high fares.

In my case, my common "B-A" is ORD-MCI. We have family/friends in Chicago, so in the past if I've had a business trip route through there on a Friday, I'll hop off for the weekend. Usually I end up with a MDW-MCI one-way home on Sunday or whatever. It's never been an issue...

The other one I would have done is DEN-MCI, but United actually has a nice (but not well-known) feature that most fares allow a DEN stopover for a flat $50 fee (or at least they did as of 2007). I've taken advantage of that one in the past. Sadly, the same does not seem to apply to their other hubs.