What is the reasoning behind some spells requiring saves and others requiring attack rolls? Is it just done to add some variety or does it have a mechanical purpose that eludes me?
I'd say it caters to different people's taste.
Well, I've always seen it as part of the way the spellcasting action takes place. Take Fire Bolt, for instance: the wizard locks his eyes on the target and conjures a flaming projectile, using his skill to control its trajectory and hit the target. On the other hand, take Fireball. The wizard simply drops it in a zone, and then it's up to the creatures in the area: if they are quick enough, they avoid it. If they aren't, they get fried. It's a passive/active dicothomy. 
I always understood that Attack is singular damage and Save Vs is Area of Effect damage. Mechanically speaking resolving those two effects requires a different mechanic.
If you roll an attack the spell can miss meaning the spell goes by the target. If the roll is a save it means it's an automatic hit the target can attempt to resist its effect.
Most ts attack roll spells are all or nothing, most save throws are all or 1/2 damage.
One incorporates AC as a factor the other doesn't...
As the others have said, it all comes down to flavour.
If a game element feels like it simulates/recreates a physical attack where your AC should provide some defence --e.g. acid arrow, spiritual weapon-- it will likely get an attack roll. If the game element strays more into area effect, intangible, or supernatural stuff --e.g. charm person, fireball, or even a grenade-like weapon like a flask of oil-- it'll tend toward a save.
No hard and fast rule either way, just a gut feel.
Emanuele_Galletto wrote:
DasFalke wrote:
I think disintegrate isn't an attack because they figure your armor shouldn't help block it, which kind of makes sense.
jaelis wrote:
I see no distinction between the two except to maintain older traditions. You could implement all attacks or all saves and meet the same goal. I hope they allow for numerous abilties for martial characters that require saves. Because the biggest problem is casters are flexible enough to choose a spell to fit the situation (lowest defense).
The_Planar_Shepard wrote:
So instead of attacking AC, the spell should target your passive Dexterity (+ Proficiency, if any) or Con or whatever. This was actually one of the things I liked about 4E.