I get what he's saying, even if he isn't saying it very well.
His complaint is that the skill tells you if someone is trying to hide something, or it tells you nothing.
He sees this as a success=knowledge/failure=ignorance issue. There's no way to ever misread someone in any positive sense. You never get "good" feelings, only a lack of bad ones, and you never get a false read, just the lack of a read at all.
And I think this can be handled by a bit of DM planning.
You're checking out the bar, looking for a particular fence. The bartender comes across as secretive, as if he's hiding something, and is actively nervous when you question him.
In a a one-track world, you could assume that he's the fence, or is working with him. Maybe he's just guilty of watering the drinks. Maybe he's just nervous because of the trouble you could cause for him by even asking such questions. He might not know of the fence in question, or of any fence for that matter, but he knows what will happen to him and his business if he's seen answering any questions for a Paladin.
You see what I mean? If the fence (or the Assassin, if you choose) is the only thing going on in the world, then an evasive answer or nervous manner tells you that he's involved. In a more complex world, all you'll pick up is that he's a shady character, not to be trusted.
In short, a false read.
A good DM could set various thresholds for the Sense Motive DC to detect not only the fact that he's nervous and evasive, but what specific points or questions are making him nervous or evasive. And there you get you "wrong answer if the DC is missed by 5 or more" result, all within the rules as written.