We used to have A-B-C-B-A routing, but that went away with multiple bases as a form of arms limitation treaty that keeps space available for smaller players. Imagine the following scenerio.
From a base in "A", an airline can also completely dominate "B" thereby controlling 2 hubs. If you imagine A to be Atlanta and B to be Chicago, you see how this gets messy in a hurry. I once had an airline that flew domestic 747's out of my base in Haneda with a long haul flight from the second position. The 747 broke even with the first small hop, the second leg was simply a licence to print money. I had at least 50 747's flying, and since they were making money on the first leg, I could land them anywhere I was interested in having market share, charging however little I felt.
Once we were allowed to create secondary bases, the scenario becomes even more insane.
From a base in 2 major markets, an airline could then decide to dominate ANY nearby airport they saw fit regardless of country. it would be very difficult to compete with an established airline, as the advice the "find an underutilized midsize airport to start in" would be irrelevant. From 2 bases in England (one for profit, one for cheap slots to get aircraft in the air), an English airline could decide to make a "focus" city out of any city in Europe.