By design, they are able to play both roles effectively, albeit with costs about wow gold. Playing too offensively will be very detrimental and often costly to your mana pool. And figuring out the amount of time you can actually play offensively is a delicate process due due to being so susceptible to attacks; Shadow fiend rarely returns full mana when used about wow gold.

There are rumors going around that Blizzard is considering alternatives to Priest mana regen other than Shadowfiend, when really giving fiend effective avoidance again is really all that is necessary to get wow gold. Pushing it beyond this, which Blizzard might do, could yield a window of offensive play that is far too long due to Priest mana efficiency being too good. Right now, you are looking at like 2-3 minutes of heavy burns and dispels before you are in trouble with mana. With the fiend buff and other rumored changes (such as buffs to hymn of hope) that 2-3 minutes might increase to 4-5 about cheap wow  Gold.