The Blazing Priest is by far one of the most useful characters in EotA. He is the best, and probably the only, true siege hero of the elves. He is great support, and can literally raise an army from the dead. The Blazing Priest, while he has no nukes, can raze bases by level 20, if you are strategic enough.
The Blazing Priest must be played both strategically and recklessly. At times, you must sacrifice yourself to the last possible moment to raze some buildings, at others you must decide which spells are more important, and you must decide at what time you must use a spell for the best effect.
The Blazing Priest's main use is a sieger, so you generally get the best spells for that purpose. His builds focus on sieging bases, not on PKing. If you select correctly, the BP can become a much more versatile hero in late game.