What game mechanic are you using to model throat slitting? I was assuming it's like an ambush, with the sleep casting roll replacing the stealth roll. The target isn't really as vulnerable as you are making out, their just compelled to wake up before taking non-mental actions.
I think the disconnect may be the result of the narrative and metagame nature of conflict resolution in the DFRPG game. Taking and giving Compels and Consequences adds a metagame element, but for players used to the more simulationist/tactical RPGs, rendering a foe helpless and then dispatching it is generally a viable option. As a result, it's hard to see "Sleeping Foe" and not figure that a followup attack would be an autokill.
That said, the DFRPG's use of consequences and such allows certain narrative advantages over purely hit-point-based RPGs. Knocking someone out in certain game systems, for instance, can be a logistical nightmare requiring large amounts of non-lethal damage and a failed saving throw, so many GMs can't really use it as a story option.
Perhaps the conversation needs to return to the context of deducing from effect.
When a Player uses a Sleeping Spell or a Tranq Dart, they want to Take Out the opposition. Whether they want to kill or simply arrest the opposition is an important choice, but one they get to make. It's their game.
When an NPC uses a Sleeping Spell or a Tranq Dart on a PC or another NPC, it's the GM attempting to create an entertaining story, having decided that kidnapping or simply tying up a PC would be a good source of tenion, or at least a way to funnel them some Fate Points.