Allowing face-to face non-FFL sales at a gun show does allow face-to-face sales between strangers that might not otherwise get together for a purchase and sale.
Although I am not a fan oppressive gun regulations, I must admit that requiring a NICS check for sales between strangers at a gun show is not an outlandish idea.
The problem with that line of thought is in defining a gun show.
For example, one version of a 'loophole' law from a couple of years ago defined 'gun show' as 'any firearms related gathering of 3 or more people', and made it a felony to be the organizer of an event where two participants arranged a non FFL sale.
So, I'm the organizer of the local smallbore league. While I'm conducting relay 2, two participants from relay 1 get to talking in the parking lot and agree to a sale of a 22 target rifle. I just became a felon (and they did too).
Heck, Grandpa, Pa, and junior go duck hunting and while walking back to the truck, Grandpa agrees to give his shotgun to Junior. If Pa organized the hunt, now all three of them are felons.
ISTM it's a hard problem. You can't just say a gun show is something called 'Gun Show', or what used to be the 'Gun Show' will become the 'Sportsman's Show'. You can't really use number of attendees either - you know that somewhere at Camp Perry, someone talks about selling a gun. Before you can outlaw private sales at gun shows but not elsewhere, you need to figure out a legally workable definition of 'Gun Show'.
It's also worth asking - even if you come up with a law that actually works as intended, will it actually make it more than 0.0001% harder for a crook to get a gun?