Which spells do folks consider worth learning or upgrading ?

[Ball Buster]

The best spells for you depends on what's your battle strategy. And that's all. In my opinion, the buffs are good in some circumstancies, like Tibster points. Onslaught only solve the initiave's problem in the first turn. Teleport or fast moving force you to lose the turn's spell, besides leaves your troop in the perfect situation to be the main target of ALL foes.

In your hypothetical situation, maybe you'd be right, but the game it's not as simply as that. In the battle, there are some circumstances that don't let your troops to do what you want. So, that's why so important the direct damage of the spells: allows you to kill some units, no matter what would happen one or two turns before.