This is one of those situations the combat rules don't -quite- cover, the stealthy taking out of combatants during a sneaky infiltration.
Sure they do. If you're the right type of character, you can pull this off as much as you want on minions, 1/day you can potentially do it to an elite, and it's possible with some extra to do it 1/day to a solo.
Also remember that as a rogue, you're quite likely to get your surprise round AND initiative on the first round. So with an action point you've potentially got 3 attacks to kill your foe in. More if you can stun him.
Knockout renders a target unconscious, which means they're ripe for a CDG. That's anything that you can bloody with a crit dead. And a rogue with a nice big attack and a nice big weapon can bloody a lot of things on a crit.
Garrote grip is like knockout++ once you get those 3 sustains in. So:
Surprise round: hit with GG
First real round (you're a rogue, so there is a very high chance you win initiative): sustain, knockout
Foe gets no action
Second real round: sustain, stunning blow IF the foe saved against knockout
foe gets no action
Third real round: sustain, foe is out for the count so you can safely drag him off and drown him in a bucket or throw him into hot magma, or whatever.
I mean sure: you used 2 dailies. But you just killed a solo elite (assuming you didn't miss or roll badly on initiative)!