DFT is designed as a late-game keystone. It can eventually get to a point where it's dealing 100 per tick. This doesn't seem like much, until you realize that it can be reapplied multiple times. DFT seems to be the most niche out of all the masteries excluding WIndspeaker's, of course. I personally don't see that as a bad thing.
Now, the reason why you'd pick DFT over Thunderlord's is due to the downtime Thunderlord's has. This downtime comes out to a minimum of 15 seconds, later on. DFT, on the other hand, if you have a frequent way to apply it, will out-damage Thunderlord's.
Swain is a good example of this. Thunderlord's is wasted on him as most of his damage is delayed. To properly apply Thunderlord's, as it doesn't stack per tick of his dots, is to E Q, than auto, or land W. DFT, on the other hand, is instantly applied by his E, reseting per tick, additionally benefiting from his E damage amp, and, will again reset on his Q. If his ult is running, it will, yet again, reset.
Basically, it comes down to this when choosing between the masteries:
Do you have a consistant way to deal damage to the enemy? If yes, go DFT.
Do you have a combo which requires more or equal to two hits? If yes, go Thunderlord's.
Both masteries seem to be designed to flesh out these two damage patterns. I think DFT is fine where it is, as a late-game, dps based Keystone.