When it actually makes sense it can be lucrative. Say you have a destination in mind and a handful of UA miles, however there are no saver awards. Instead choose to sell at 1.5 cpm and book on a different carrier for a cheaper cost and pocket the additional cash. Technically a win as

1) You already had a destination in mind
2) The miles were 'worth' more cashed out than redeemed for a non-saver
3) You get MQD/PQD on the other carrier

All depends on the flexibility you have and the way you want to use your miles. If you want to lock down on a schedule, at times it is a better overall value to sell.

However of course, realize that points brokers are in this for the money as well, and they profit very nicely by using these miles and selling trips to in First/Biz to Asia/AUS and the like for a very high margin. All depends on the eye of the beholder.