You only gain that time effectively once, and inbetween thresholds you slowly lose it again until you get another tick to add duration back on to the DoT. Don't forget that haste makes the time between ticks smaller, reducing total duration. If it didn't DoTs dps would just become longer durations for more fillers without getting a DPS increase themselves. That isn't the case at all as revulva points out. If you're trying to say that each time you meet a threshold you gain more time to do other things, show me where UA's duration gets longer than 17 seconds.
Ok Shumoo, you've posted alot of wrong information in this thread already, please don't confuse people more with your misinformation. EVERY haste treshold you get the dps gain from an extra global cooldown every so many seconds.
Assume a theoretical 1680 second fight with 50% haste, that means 840 ticks and UA has 8 ticks per cast.
For every tick we have gained upto that point, we received extra global cooldowns:
5 tick UA = 168 casts
6 tick UA = 140 casts - gained 28 GCDs
7 tick UA = 120 casts - gained another 20 GCDs
8 tick UA = 105 casts - gained another 15 GCDs
every extra tick you gain, you have to cast less. The dps gain from the previous extra tick does not go away and you get an extra dps gain each time you receive a new tick. However YES this dps gain becomes smaller and smaller for higher haste tresholds. And YES as stated by Revulva, these extra GCDs are really not that much of a dps gain and the whole discussion about it got quite out of hand.