A few days ago I decided that it would be fun to join my wife on her business trip to DC. She already had her trip booked on United and I wanted to join her on the outbound flight (for various reasons we'll be on our own for the return). The round trip fair for this trip, for me, would have been close to $500. This is more than I wanted to pay for a short trip. After a bit of investigation, though, I found that I could make this much more affordable by splitting the trip into two one-ways:
For the flight out to DC, I was able to use my small bank of United Miles to book a one way trip on my wife's United flight (only 10K miles for the one way!). For the return trip, I found a one way on Delta for only $69. I could have saved a few more dollars by flying Southwest, but my status is on Delta and so I always welcome chances to earn more MQMs.
So, the point is that even though airlines usually discount round trip fairs it sometimes pays to look into one-ways instead.