You are right about the frogsac not being very wearable in an active setting. Basically, once you get everything set up, and you just want to sit and eat, the frog sac is wearable. too much movement and the toggles will come apart. it is better to understand this as a wearable blanket than a jacket/sleep bag.
However, the frogsac is longer for us folks who are 6'5, cheaper for those who are poor. I am not sure what the NS temp rating is as a stand alone bag but i assume it is lower than 40 deg?