I do note, in your example, you chose to use an NPC from a module you liked which had an ability that sounded very much like speak with dead and the NPC would use this to get plot information necessary to the overall goal. I'm just still befuddled why PCs couldn't / shouldn't be allowed to do this as well or how its a "bad thing"?
The short answer: for much the same reason that the players can't start play as emperors or gods.
Now I'm not saying that an RPG can't be run in which the PCs are emperors (or gods). Nor am I saying that this couldn't be done in D&D (maybe there is a reason why the emperor has to pretend to be a commoner, and doesn't and can't access his wealth, armies, courtiers etc). But I don't think one can just
casually permit a PC to start the game as an emperor (or god) and not expect it to have a pretty big effect on the way the game plays.
Whereas NPC emperors, gods, and speakers-with-the-dead
are quite OK. They are various sorts of plot device that I (as GM) can introduce into a situation as I do or don't see fit. The players can also introduce these elements into a situation - by successfully deploying the resources at their disposal (mostly their PCs and those PCs' relationships) - but the adjudication of these situations is ultimately under my control as GM. (So, for example, the benefits of successfully dealing with the emperor to gain access to magic items can't be any greater than the standard treasure parcels for the level of the PCs in question. The benefits of using Religion skill in a skill challenge to commune with one's god can't be greater than the benefits of using History instead - although they might be
different, sending the scene in a different direction from wher it might othewise have gone.)
Ultimately, if the speaker-with-dead NPC becomes a liability to the game, I can kill him off. This is not so with a PC.
From my experience, it makes them more central to whatever quest they may be working on if they don't have to haul around the DM plot device. Now demons might seek to kidnap one of your players instead of the plot device which would create a very dynamic scenario. To me, thats what you lose when you take away those "I win DnD spells" whcih some people find useful to their stories and others find annoying.
I'm not a big fan of hauling around GM plot devices either. If the PCs want to talk to the speaker-with-dead they will have to go and find him again. Unless something changes radically in the game, there is zero chance of him accompanying them anywhere.
As for demons wanting to kidnap PCs - that may or may not happen (it happened in my last game, in which one of the PCs - a fox spirit - was violating the terms of his banishment from heaven, and so constables of hell came to arrest him; in my current game, one of the PCs is a demonskin adept and enemy of Lolth's cult, and so is certainly in danger of attracting hostile demonic attention). But I can introduce that sort of situation into the game without needing to introduce the "game-breaking" magic as well.
the PCs feel the game world has more life to it if the world revolves whether they are involved or not. If larger events transpire and they can either choose to get caught up in them, avoid them, or sometimes are mercilessly swept up in them, it gives the feeling that they are a part of something substantive which they can interact with but not necessarily control.
Whereas I would only tend to mention such larger events to the players if those events were of some significance in helping establish the situation in which the PCs are engaged, or in enriching the context for the players to make choices for their PCs. An analogy, not exactly perfect but near enough - if it only deserves mention in the Appendix B timelines in LotR, and not in the actual text itself - like, say, the battles fought by the dwarves of the Iron Hills - then it's probably not going to get mentioned by me to my players. Or, to put it another way - I engage my players in the world by using myth and history - which gives shape and context to the events they are actually engaging with via their PCs - rather than current affairs, unless those current affairs also contribute comparable shape and context.
This does have consequences that some don't like - for example, if an NPC or an event is mentioned in any detail then the players know that
adventure lies this way - but I'm from the school of "no need to search for the fun". Again, others differ (in part by rejecting that description of sandbox play).
Heh, good one!
