I very much agree with the part in bold, and I think it’s one reason it’s so hard to handle lying well in an RPG. So hard, that although I rail against using Insight as a binary lie detector test, I understand the appeal: all the other options are either really hard to do well (requiring either fast thinking or preparation time...or both) or they are generally transparent, nudge-nudge-wink-wink euphemisms for binary lie detection.
So, yeah, I get just wanting to avoid the whole thing with a simple, mechanical solution.
A point I didn't make clear is that even with the responses in my example, there's no guarantee that Franky is lying. If I go back to my example
PC: "Where were you last night after dusk."
Franky: "I was at home as usual." [he's being sarcastic and condescending]
PC: "Any witnesses?"
Franky: "Nah, I live alone. Nobody but me." [shift's a bit in his seat, this question makes him uncomfortable]
PC: "So you know nothing about Jimmy the Nose dying?"
Franky: "Jimmy's dead? No I didn't know that" [not really surprised]
Franky: "I was at home as usual." [he's being sarcastic and condescending]
PC: "Any witnesses?"
Franky: "Nah, I live alone. Nobody but me." [shift's a bit in his seat, this question makes him uncomfortable]
PC: "So you know nothing about Jimmy the Nose dying?"
Franky: "Jimmy's dead? No I didn't know that" [not really surprised]
Maybe Franky is just a sarcastic SOB who doesn't like being questioned. His girlfriend just dumped him, but he doesn't like to share that so it makes him uncomfortable. He didn't know that Jimmy was dead but he's not surprised because Jimmy pissed off someone he shouldn't have. On the other hand maybe Franky killed Jimmy which would also be a perfectly logical reason for his behavior.
Depending on the scene and what I want people to know I may just act this out broadly enough that anyone should be able to pick up on it. But if I view Franky as simply being someone who habitually lies and tries to hide his true intentions out of habit then an insight check might be appropriate.
So like I said earlier, insight is not a lie detector or mind reader but in my games it can give clues to attitude and reactions.