I'm having trouble seeing why it's better to treat this in a binary manner. Either certain divination spells/rituals (and really, this applies to any class ability) don't exist at all in the campaign, or they exist and always function.
Isn't there a middle ground we're excluding?
There is certainly a middle ground where scrying doesn't work due to an NPC ability, say, to block said scrying. Or, say, a % chance that scrying will always fail.
Let's look at a system where the players know that there is a 50% chance that scrying will fail. The PCs pay X resources to scry, and then roll the dice. If their scrying fails, they then have a meaningful choice -- should I pay X again for another attempt?
Let's look at a system where NPC abilities include a spell to block scrying. The PCs attempt scrying and it fails. The PCs now have meaningful information -- the target is concerned about scrying, and the target is powerful enough (or has powerful enough friends) to cast this spell. If the spell is high level, the players also know that at least one spell slot is used (in a Vancian system).
Let's look at a system where the scry fails, but the PCs do not know why. They have the option to try again, but they have no reason to believe that their actions can make another attempt succeed. They are now presented with a mystery that the DM ought to be able to explain, which they ought to be able to uncover when they defeat the villian.
Finally, let's look at a system where the DM simply nerfs the ability, and tells the players that they weren't specific enough. Doesn't that encourage the players to spend X resources again in an attempt to be more specific? Doesn't that take any meaningful benefit from the ritual (meaningful choice, information, mystery) from the ritual, leaving the PCs penalized for using their abilities wisely?
I've said it before, and I'll say it again: Good game design/advice doesn't include making smart play opposed to satisfying play. Good advice in a DMG is advice that makes smart decisions satisfying to the players. Sometimes that means that an adventure cannot proceed the way the DM envisioned it, and that is absolutely okay.
Actually, in many ways, it is desireable. It is usually when the DM is pushed out of his comfort spot that events occur in the game world, for good or ill, that are remembered long afterwards. IME, on both sides of the screen, anyway. YMMV.
RC