If you go purely demonology to 50, you'll gain Rohan Gold the support of a pseudo-warrior pet capable of tanking and dealing massive damage all at once. The demonology tree is well rounded and very useful to first-timers or those favoring the more demonic side of operations. Destruction is the build of choice in PvP and end-game raiders for consistent high DPS; it's also a hell of a lot of fun when blasting away people in Arena and BG's with massive shadow bolt critical or a conflagrate.