So the way I see it, there are two parts to this question then:
1 - Can you just drop the second leg - and 2) can you get a refund on this second leg
Can you drop the second leg of a flight
I've done this once, and I assure you, it's possible. However, there are several factors to consider:
say you have a flight with several legs - five for example. If you miss the second leg, depending on the conditions of carriage, the airline will often cancel the other legs - after all, you in theory won't be there to catch the third, fourth and fifth legs. And you almost certainly won't get a refund, unless their conditions of carriage include refunds for emergency situations, which you'd have to prove.
if your flight has only two legs - from A to B (maybe with stops), and from B to A, and you miss that second half - B to A, you can do it, but again it depends on the conditions of carriage. I did this once with a flight where A and B were London and Buenos Aires. I'd worked my way up to Colombia and decided to hop across to the US and fly back to London that way, so just dropped that second leg. And that was fine. HOWEVER, and this is important - some have an extra clause where you can be PENALISED for missing that second return flight. So it's VERY important to read those terms and conditions carefully.
a friend had to get a flight from La Paz, Bolivia to Santiago, Chile. He found that it was in fact CHEAPER to buy a return than a one-way (this happens occasionally). Fortunately, once more he was able to do this without penalty.
in addition to possible financial penalties, there are some other possible penalties. If you're a frequent flyer member with the airline, it's possible they may cancel this. It's also possible they may refuse to fly you again. It's rare, but remember, they're usually pricing flights like this for a reason, and you messing with their plans doesn't go down well :)
Can you get a refund on the second leg
Again this will depend on the airline, and reasons.
You almost certainly won't get a refund, unless their conditions of carriage include refunds for emergency situations, which you'd have to prove. A friend did this on an emergency trip - he cancelled the second leg of a different flight he'd have to take.
if for some reason the flight time gets changed to one you can't take (even though you never intended to take it) you may be able to argue for a refund.
If the flight was somehow not as described, you may have a right through your travel agent for a refund. The airline, however, probably not.
in general, flights are priced as such because they don't want people taking one way flights. They want them taking two. Otherwise there's unnecessary airport taxes, staff costs and all that, for potentially empty planes coming back to pick up the people on a popular route. Sometimes it's also to encourage you to come to their hub. For example, it could be worth Qantas's while to give you a discount on a leg into Australia, because all other flights you take internally are likely to be on Qantas then, and they can make their money back. For comparison, a return flight from SYD to LON is generally more expensive than a return flight from LON to SYD.
Bottom line - read those conditions of carriage. Again. And again. Make sure you won't accidentally shoot yourself in the foot by dropping that leg. Make sure you CAN get a refund, if that's the only way it's worth doing for you. Check with your travel agent, if you're using one. Then book it and enjoy!