James McMurray said:
I'm not talking about from an NPC on the battlefield perspective, I'm talking about from a GM perspective. There are many times when destroying a spellbook, holy symbol, etc. is a great idea. Yes, it might not be the best option when you're waving a sword in your face, but unless the game is very one dimensional, things happen outside of sword waving situations.
If you're destroying the fighter's gear but leaving the wizard and cleric alone, there may be something imbalanced in your setup.
I agree there are times when these are valid options...but IME its fairly rare when it's actually actionable or valid except in some broader metagaming sense and does nothing to enhance the game, IMHO. With the exception of the wizard's greater vulnerability, spellcasters are far less exposed to loosing their abilities and the fighter's loss of efficacy is relatively minor.
Destroying a Wizard's spellbook is a possibility, but rarely within the confines of combat. For one thing, it's usually valuable loot...and unlike a magic sword, it isn't actively hurting you. Destroying a spellbook isn't going to stop the wizard from casting Finger of Death...but getting up in his face WILL. All of that's assuming he's even carrying it on him, which he may not be. And if you're close enough to inconvenience him, you're better served hitting him than killing his equipment. The holy symbol is a better example (as is the component pouch), but again, these are not universally required for every spell. And at the cost of 1 gp* or 5 gp respectively, it's safe to assume most spell-casters would carry spares...or would start doing so after the first sundering. Just like the fighter carries a secondary weapon or scavenges one in a pinch.
Outside the battlefield situation, it's very much a non-issue. Unless the cleric is practicing a very unpopular or non-standard religion, acquiring a holy symbol or fashioning one is a fairly trivial task. Likewise, for a spellcaster, the rules are specific in that anything but g.p. valued components are to be considered too trivial to track (and likewise trivial enough to replace). So even if you destroyed them in every session, it wouldn't be the same as destroying the fighter's primary weapon.
Destroying the spellbook is expensive and penalizes the player in a way destroying the fighter's weapon does not. Destroying a +3 Hammer of Fiery Burst means the fighter looses some nice bonuses. Destroying a spellbook if the wizard has no additional copies reduces him to either creating a new one (which results in downtime for the character), memorizing from another spellbook (if one is available, and rolls are successful) or doing nothing with the character for a while. Sorcerors and bards, of course, have no such issues. In general, destroying the spellcasters spellbook is a trick you pull ONCE. Because it has a chilling effect on a game or simply pushes the spellcaster to spend more money on protecting that investment, and thus returns to the non-issue status.
Either way, we return to the 'rub some dirt in it and get back in there' idea, IMHO.
* - (unless you're one of those RICH clerics, who opts for the fancy-schmancy 25 gp Silver one, of course)