It seems common that people build chars that compliment themselves. Like air/water and fire/earth wizards. But when they do so it takes 2 turns to get the effect you want, and the enemy moves around. So better to have one char be able to setup, and another to unleash. Like this you can do it in one turn, a wizard chasts oil, and a ranger uses a fire arrow for example.
It only takes two turns if you don't have the AP do do it in one.
This is admittedly the case for most characters unless they have high speed, but if you are willing to run a glass cannon mage on the back line, and I really don't know why you wouldn't do that if you are running four characters, he'll easily be able to make both effects in one turn in any turn but the first one from early in the game.
(If you are running hard difficulty, Glass Cannon hurts a lot due to the additive stacking of health modifiers, but if you have four characters and the mage has rudimentary defensive spells, you can hack it.)
Its the same thing with a buff like Oath of desecration, if your warrior or rogue is casting it on himself, that is a whole turn out of 3 or so that is spend. If another char casts the buff the damage dealer can get full use of it. Same with haste, you will often want someone else to cast it to be able to use it in the same turn.
While I agree that it is very practical to have somebody else in the role of buffing a frontline warrior, Oath of Desecration is only 3 AP and can easily be fitted into a warrior's AP budget.
If you are going to have your warrior being a warrior-mage with 8 int (for 100% cast on the lowlevel buffs), it would be silly not to spend a single point on witchcraft to pick up Oath of Desecration.
