Players creating new spells?

[The Madhatter]

I think the reason is just a little more practical and real world. What if the PHB did contain all the spells ever written in GH? We've seen the 2e tomes of spells and they were amazing to behold! I think the designers went through and tried to get the "critical for gameplay" spells included and groupls rely on other sources and player ingenuity to flesh out the rest of the spells at large.

 

[Al]

In my campaign I do attempt to move some way towards reflecting magical "research". Essentially, all spells in the PHB, T&B and other WotC accessories are common knowledge, available to all wizards and sorcerors. I also have, however R&R and R&R2. Given that these are generally regarded to be slightly more powerful than standard PHB spells, each spell is associated with one or more of the Great Colleges of Magic, with each of the four major nations having their own particular magical specialty.

 

[Humanophile]

I'm pretty sure that there are rules for players making new spells, possibly in the DMG, but there are two, maybe three reasons against it.

First, both in game and out, the spells in the PHB do most things that someone would want to do. Making a new spell that's of general use would require finding a new "niche" for an effect, and that's kind of tricky. (Two exceptions exist here; one is "nonadventuring" spells, which you can assume exist and just don't show in the adventurer-specific PHB to save space on things the players aren't likely to do. The second involves making a specific use spell, such as "teleport to one specific location" rather than the more general Teleport spell. In this case, the spell is likely only one level below the general one, and since its shelf life is likely too short to make it worth the investment of time and energy, they often only exist as specialty scrolls to avoid overinvesting in underlings.)

The second is mostly out of game. Balancing a new spell can be a pain in the ass. (Imagine we didn't have 3.0 haste to go on as a guide. Imagine a player wanted a spell that gave him an extra round's worth of actions. Think how easy it would be to undervalue said spell and throw everything off.) Creating a new effect is easy. The hard part is giving it appropriate drawbacks and chosing the right level for it. So in-game it should be a major investment of time and effort, since you don't want the players repeatedly taxing the DM's time and effort performing a large number of balancing acts.

And third, spells in the PHB are likely the most "efficient" spells possible; long-distance area-effect damage is in line with third-level effects, so letting a character make a second-level spell that's like Fireball, only with shorter range and smaller area, is a mistake. And characters are unlikely to want to make a spell that's like Fireball, only higher level, so the impetus to create on that end is diminished. You want something that works like a PHB spell, use the PHB spell (or something with only cosmetic alterations thereof), trying to reinvent the wheel will likely result in your creating an "inefficient" spell that's a level or two above the power level of a PHB spell.