I have noticed that a lot of published adventure materials (as well as the Core Rulebook) instruct that opposed rolls be made:
Perception vs. Stealth
Sense Motive vs. Bluff
But aren’t they actually unnecessary? Can’t we just treat one of the “rolls” as a flat DC?
Let’s take a Sense Motive roll as an example. An NPC tries to lie to a PC. For simplicity’s sake, let’s assume the NPC has a +0 Bluff modifier, and the PC has a +0 Sense Motive modifier.
If you treat it as simply a DC 10 skill check (DC 10 + opposed skill modifier), then the PC has a 55 percent chance of succeeding. I don’t think this changes with opposed rolls.
To illustrate further, let’s say we want to invent a dice-rolling game in which I have a 50% chance of winning. Either you and I can each roll a d20, and I win if my roll higher than yours and we re-roll if we tie. But what’s the point? I might as well roll a single d20 and win if I roll 11 or higher. (Or flip a coin!)
Basically, rolling two d20s seems like doing double duty. And since skill checks are not automatic successes on a 20 or automatic failures on a 1, nothing is added or subtracted to the possibility of success by rolling d20 twice.
I see this also applying to Perception vs. Stealth rolls. HOWEVER, it doesn’t necessarily work when the skill you are trying to “beat” is being practiced by multiple creatures. I suppose the rules intend the party to detect the LEAST stealthy individual among a group of hiding creatures, thus adjusting all the probabilities. However, as a GM I skip all this because it’s too much work for little payoff (and it makes Stealth checks too easy to defeat) — and so I just have the PCs do Perception checks against a flat DC.
Conversely, if an NPC is trying to detect whether a PC is bluffing, I don’t see why the PC should just try to do a Bluff check against a flat DC based on the NPC’s Sense Motive modifier.
So I don’t see where opposed rolls are necessary. Or am I missing something?