To add a bit to this discussion
a) I agree with the various posters who have no ethical qualms with hidden city and point beyond ticketing. They can use whatever mechanisms they wish to force me to pay more for less; and I will use whatever mechanisms I can to pay less for what I want. I've paid 99˘ for 2 liters of Coke instead of $1.29 for 24 ounces many times, and no convenience store has ever come after me for the difference.
b) As far as the OP's question: I've done this a few times and haven't been caught. I wouldn't suggest doing it every trip (at least, if you relish your miles/status) but once in a while they don't seem to catch on. For folks in MSP, for example, trips through MSP to other cities (MSN, MKE) are often considerably cheaper than the direct flights. Last summer I booked a o/w through MSP to MKE which was $300 cheaper than the direct flight—and actually got mileage and EQM credit for the last leg! As far as NW/DL knows, I was on that plane. I just booked another—then end of a several-legged trip where I needed to book a o/w to MSP. They wanted $700 for that, but I found one to MKE for $230. We'll see if I get credit for that, too.