Though some priests have found a use for the discipline tree, right now, it doesn't quite have a solid purpose/goal, but rather seems to act as a supporting tree. This is something we plan on addressing with Wrath of the Lich King. Originally, the goal of the tree was primarily intended to provide staying power, vs. the holy tree which provides throughput.I'll admit that my own priest is sadly a mere level 8, but I'm aware that this has been one of the big priest issues for a long time. One of my friends strives to be one of those special priests who have "found a use for the discipline tree" and he seems to do pretty well at it, though he admitted that he recently changed some of his talents around, leaving behind lesser-used talents in discipline in exchange for some holy talents. He refuses to give in and become yet another shadow priest, since he revels in succeeding at something few other priests try.
Also, so you're aware, the 41-point discipline talent, pain suppression, is being changed to be usable on others for patch 2.3.
I would be quite interested to see how Blizzard changes the discipline talents in the next expansion. Will they stick with the original "staying power" idea, or will they go for something new? What do you think needs to be done?