Wizards have access to 2 very similar spells: Shield and Mage Armor
Both provide you with AC +4 Bonus, but Mage Armor has Range touch (i.e. you can cast it on other party members) and Duration 1 hour/level, while Shield has Range personal and duration 1 min./level
From my point of view Mage Armor is far superior to Shield. In my party Wizard and Monk formed mighty combo, where monk got +4 AC bonus for free every fight. It looks like a gamebreaker.
Question: Why are these spells, which seem so similar in effect, so different in range and duration? Are there any drawbacks or advantages to one spell or the other which I didn't see?
Best Answer
Armor bonuses are more common than shield bonuses.
Mage armor provides an armor bonus to AC, which doesn't stack with other armor bonuses. Since nearly everybody has an armor bonus (even proficiency with light armor gives access to mundane options that match the +4 bonus), this has a limiting effect on its usefulness to non-caster classes. This is why the spell is allowed to be cast on anyone and to last so long: it's not as useful so opening up the target range doesn't have as big an impact on balance.
Shield, on the other hand, provides the much less commonly-used shield bonus to AC (many fewer classes have this proficiency, even fewer of them usually take advantage of it, and getting a mundane shield with a +4 bonus is a build-defining choice). Very few classes run around with a +4 shield bonus, so even those who do use shields would find it very useful... and thus its availability is limited to the caster, to keep that usefulness in check.
It's the same reasoning that makes rings of protection (deflection bonuses) cost twice as much as bracers of armor (armor bonuses) with the same AC value: the less common a bonus is, the more valuable it is because it's less likely to conflict with another existing bonus (which would make it useless).
Two asides
It's worth noting that mage armor and shield are both force effects, which have various usefulnesses, but that only shield stops magic missile. It appears Wizards overestimated the value of direct-damage spells, and underestimated how easy ranged-touch-attack spells would be to land, so they thought magic missile would be a cornerstone of wizards everywhere. If that had been true, shield would be much more useful--making its duration/target restrictions more sensible.
Exactly how bonuses stack (especially as regards armor bonuses and enhancement bonuses to AC) seems to be rather murky and contradictory in the rules, so if further clarification is needed on that particular aspect of this issue, it should be made a separate question.