Just tried it myself with SEA to MSP (hopping off on a SEA->MSP->DFW route) on July 15th. Direct flight is $230, hop-off route is $150 on exactly the same Delta plane.

Now the thing is, if I book SEA->MSP->DFW for Friday and MSP->SEA for Sunday I don't see how Delta could not notice this and block me from flying because of abusing this system.

Actually, I have done something like this before. When I moved from Europe to the US I booked a return ticket because it was 500 Euro cheaper than going one way, knowing fully well that I would never be on that return flight.