Booking a return to "take advantage" of the offer and then only using one-leg would be pretty stupid, IMHO. You''ll be hit with double the tax/fuel charges that you would pay by getting a one-way ticket outside the offer and it will cost exactly the same number of miles.

The only advantage for you would be if you think that you might possibly use the return on another trip (by combining it with another one-way out there). remember that once you have flown the first segment of the return ticket, you won't be able to change the return date though.

Excuse me if I have misunderstood.