I wouldn't really count on a Berserker, and even less on controlling him. With a meager spell power, a Knight won't be able to cast many spells; people generally prefer mass spells over high-level single-target spells.

I generally agree, except for the Berserker; spell power isn't that important there, the main thing is to have skilled Dark Magic.
Dark Magic works very well against Orcs, and, as already mentioned, against Demons. But in Heroes 3, Light gained a significant advantage: Divine Retribution. It's a very powerful spell... although it's not guaranteed that you'll get it, but still. ;) When choosing between Light and Dark, you need to consider the opponent's race and the spells available in the guild: for example, if you have Weakness and Feeblemind, it's more advantageous to take Dark Magic with a mass curse, and if you have Bless and Holy Fury, then Light Magic with a mass blessing.