A-B-C-B-A existed in a previous version of the game, and it was massively exploited by big boys: they were in reality creating phantom hubs and crushing everyone else.
However, filling a cargo plane is not necessary to make good money. Cargo is very lucrative, and my rule of thumb is that a plane with 60% load factor on only one of the legs is usually enough to break even - the rest is bonus (on only one of the legs because cargo is usually very asymmetrical).