Here is an another house rule from the Wizard campaign, whose purpose is to add an extra dimension to spells, both making them more flexible and to reflect the difference between mages and commoners. Think of Gandalf’s imposing figure, or the effects of wizards upon their surroundings in Ars Magica. And you can think of M:tG, where spells have a minor secondary function.
Against non-spellcasters a spell can be converted to a bonus to a social skill. The bonus is +2 pr. spell level of the spell converted. The duration of the effect is dependent on the situation, and in most cases it covers one skill check, but there may be situations, where the spell covers multiple checks.
When converting the spell it is still considered cast as normally, but instead of applying the normal effect, the spell is instead used to support the spellcasters social skills (bluff, diplomacy, intimidate, sense motive).
A wizard uses his Burning Hands spell (lv1) to terrify an innkeeper. This grants +2 on the Bluff check, as he wants to fool the innkeeper in believing he is a powerful mage.
The Sorcerers spends her Hideous Fanged Mutilation-spell (lv2) and gains +4 on her intimidate spell, when trying to stop the mob of angry villagers.
Dealing with diplomat the enchanter spends his See Invisible spell (lv2) to gain +4 to Sense Motive as he becomes more sensible to tiny gestures and otherwise hidden motives.
The player chooses the spell and converts it to gain the modifier. The net result is, that even if the players memorized all kinds of offensive and defensive spells, they can get some extra mileage from them. The spells also boosts areas, where the wizards are not very skilled, and this enables them to function more easily. It is ofcourse important to remember, that this does not apply to other spellcasters, as they are aware of what is happening.
Why my players enjoy the rule
For my gaming group the benefit of this rule has been, that it emphasises the difference between wizards and commoners, which is an important difference. It allows the players to use their spells in social situations of which there are many, and yet they have to be careful. The fireball is good at intimidating commoners, but perhaps you’ll be needing it for some other things later? In addition it makes it easier for the players to use their offensive spells without worrying about killing innocents, e.g. the Hideous Fanged Mutilation-spell is a terrible spell, which easily kills your average commoner, but there are times when the wizard just want to chase the commoners away, not kill them [also in this campaign no XP is gained from killing opponents, so you’re not tempted to kill everyone you encounter].