Let me tell you a story about PvE mana burn. It's about a little instance called "Zul'Gurub". If you weren't a raider at 60 you probably have only heard of it in vague, knowing references, but it was a big deal in those days. It was the first 20-man raid. In relation to Molten Core, the entry-level 40-man raid, the fights were more complex, the gear requirements lower, and the loot at least comparable. All this meant it was a lot more fun, and a lot of people ran it.
Well, several bosses in that instance are casters. Some of them have truly nasty spells and effects that you *dreaded* seeing. So some people got an idea: "Hey, if we mana burn these bosses, we can reduce their spellcasting!"
So in several fights, some guilds tried using mana burn, viper sting, and mana drain to inhibit the bosses using their abilities.
The result was the classic "mixed bag". In some cases, it did _slow_ the rate at which the bosses used their abilities. I emphasize the word slow because it in no way stopped the bosses from doing their thing. Data mining revealed that bosses often need very little mana to use their spells. So while it was easy to knock off MOST of a boss' pool, it was nigh-impossible to keep them down to the point where they stopped casting. NPC mana regen, after all, is independent of the 5SR (as PC's used to be, but that's another story); it always ticks. Keeping a boss at zero mana proved to be extremely difficult and, at best, terribly inefficient.
Jin'do the Hexxer was the worst. Jin'do has such a bewildering array of abilities that even slowing them down was seen as a vital component of the fight. Unfortunately, not only is Jin'do's mana pool massive, but the fight is an endurance fight by nature. Priests in particular were forced to spend a prohibitive amount of mana trying to burn him down, meaning that the latter portions of the fight were terrible.
After a while, most guilds gave up on trying to mana burn even those bosses, and used their mana and time towards killing the boss/ healing the raid.
And thus: Mana Burn = PvP spell. Don't use in PvE.
There is only one fight in all of WoW where mana burning is a viable PvE strategy, and that was in AQ against the Obsidian Destroyers. If you're not in AQ, don't think about it.
Edited, Feb 7th 2008 7:10pm by ChahDresh