What the airlines are really trying to eliminate (mostly by penalizing the already haggard travel agent) is back-to-back throw aways and hidden city ticketing. Examples:

I decide to go see Catman in beautiful Jerzee next Sunday, 12/20 and return on Tuesday, 12/22. Fare? $1644. Instead of this exorbitant fare, I book a departure on 12/20 with a return on 12/27. Fare? $513. I book a ticket originating in beautiful Jerzee for travel 12/22 and a return who cares when. Fare: $513. I throw away the returns on both tickets, see Catman (and beautiful Jerzee) and save $618! Or from the airlines point of view, cost them $618, tie up four deeply discounted seats when I'm only using two, etc.

Hidden city example:

I need to go to Denver next Sunday. Fare? $279. If I book the same flight as a connection to Kansas City and just get off the plane in Denver fare is $224 saving me $55.

There are literally thousands and thousands of these "opportunities" out there. The airlines have created the mess with their myriad of fare structures. Good travel agents used to take advantage of these but it has simply become too expensive for them should they be caught...

Hope this lengthy ramble is helpful...