Its not against a DC is Naga's entire point.
The roll is to demonstrate to a player how skilled the character in question is.
Its so a player, who may be curious what the character's speech, action, etc has accomplished has some way to gauge it beyond what that player merely wrote or emoted.
There is a difference between saying
*describes Jake Black in perfect detail*
because if you do that I ignore your description
and
"describes a dark haired man, heavy set with strangely expressive dark eyes, longish hair, and white skin" *roll spot check* ==29
Now I know you're character got a really good look at him, and I may now ask you just who you're describing because damn--that's a good description. No DC required.
OR
"Buy my grapes. You really like grapes. I can tell you're hungry, and they're mouth watering."
I'm not buying your damned grapes. OOC I know they're useless because there aren't even hunger rules enforced on EfU, and I've got enough RP junk food in my inventory. Plus, frankly, that wasn't persuasive in real life--even though it was the absolute best that Joe the Frank seller's player could muster.
But
"Buy my grapes. You really like grapes. I can tell you're hungry, and they're mouth watering." *roll persuade check* ==3
Confirms that that was indeed the world's worst pitch line and I'm happy to ignore you still.
Meanwhile
"Buy my grapes. You really like grapes. I can tell you're hungry, and they're mouth watering." *roll persuade check* ==22
Tells me that while the player isn't persuasive, the character obviously is. The guy is only asking 1 gp for his grapes, its not a big deal, I'm not forced to buy them, but there's no RP reason to refuse so why not?
OR
Begger: "Give me some coins and watch my juggling act!"
I may leave a coin.
While
Begger: "Give me some coins and watch my juggling act!"
*roll perform check* ==16
*roll tumble check* ==19
It was a pretty good performance. I may leave 5 coins.
Meanwhile
Begger: "Give me some coins and watch my juggling act!"
*roll perform check* ==16
*roll tumble check* ==19
Begger: *tosses multicoloured balls in the air, catching them while dancing on one foot, the balls whirl quickly in a fanciful dance around his head and his curly white mustaches as he spins and laughs his heavily patched cloak spinning around him like papers in a wind*
Damn that performance rocked, I'll leave 10 coins now.
Of course you might go:
Begger: "Give me some coins and watch my juggling act!"
Begger: *tosses multicoloured balls in the air, catching them while dancing on one foot, the balls whirl quickly in a fanciful dance around his head and his curly white mustaches as he spins and laughs his heavily patched cloak spinning around him like papers in a wind*
I'll now go, wow, he emoted that really well. I'm curious if he's even got any perform or tumble skills--I mean he's a dwarf with an 8 DEX....hmmmm...guess I'll just leave a coin.
The rolls can simply enhance for you what the rp of the character is saying.
It is, truly, no different than the following.
Thief: "Give my your GP or your HP!"
Random Guy: "No. I'm not intimidated by you!"
Thief; Rolls Initiative 20+15=35
Thief: Rolls Attack 20+5=25 HIT
Thief: Damages Random Guy 64 Points
Random Guy Dies
There was a roll. You were forced this time to abide by its decisions. You now roleplay a dead character.
In my mind, its far freer to admit that maybe that guy who rolled a 66 on his persuade talked me into spending a single gold piece to buy his grapes--if I really don't want grapes I can still freely ignore him, but I will also realize he's very persuasive and roleplay my response accordingly instead of "No grapes, fuck off!" now I'll just say "I really wish I could, they look delicious and mouth watering, but I simply haven't the gold to buy them. I'll come back when I can afford them."
Of course, maybe he'll respond--"Well bitch, I just rolled a 66 on my persuade and you blew me off. Now I'm rolling my 35 init and my 25 attack roll and my 64 damage. When you're dead, I'll just take a gold coin and leave a grape in your inventory."