When I first seriously started nosing around roleplaying games in 2002 or so, one of my first stops after Sorcerer’s Place’s Baldur’s Gate sub-forum for pen and paper games was Wizards of the Coast’s own forums. I didn’t delve deep or for long, because most of the discussion was opaque to me, not being familiar with the rules of Dungeons & Dragons. A few things leapt out at me at the time, and still stick with me as being curious.*
One example in particular came up in the section meant for GMs to discuss their tricks and quandaries. A poster related the story of how, during play, a player asked if their wizard character could tear a spell out of their spellbook and cast it as a spell scroll — i.e., it would burn up or fade away or whatever it is scrolls do when they’re used. In the context of the story, it was an emergency, last ditch effort. At the time, I thought it was a wonderfully creative thing to do and didn’t understand why anyone would object to such a notion.
To be honest, I still don’t fully understand the reasoning behind the objections at the time. Yes, it sets a precedent of casting spells one hasn’t prepared by ripping them out of one’s tome of eldritch lore, but it comes at the cost of not having the spell anymore, as well as having damaged the repository of your arcane might.
I probably still don’t fully understand the economies of spells and spellcasters in Dungeons & Dragons, but at that time and still now, I think it’s a fantastic idea and would totally want to be able to say yes to a player who’s got that gleam of desperately grasping at any straw to pull victory from the maw of ignoble failure.
* Another thing I didn’t get then and still don’t get now were the myriad objections to the so-called SPUM, or Spell Point-Using Mage.
Held Action 
Good thoughts. This is part of why in the game I running the spell casters are very very open ended. They have a plane of influence (eartth, air, water, lightning, etc) and a power level. If someone has a level of 3, they can cast 3 spells. A power level 1, power level 2, and power level 3. If the player has power level 5, then that’s 5 spells, one at each level. This requires the players to think about power, effect, and limits resources. The spells reset after a given period of time (Such as a battle or a day or a difficult trip). I want to see and reward creative spell casting, not put limits on what can or can’t be cast outside of their actual character.
I like the idea of casting spells from a book as scrolls. I would even go so far as to allow a spellcaster to cast spells from another wizard’s spellbook in a similar manner.
Peace,
Christian