Rescue is very useful when you're fighting one mob (or multiple mobs, with just one fighting the tank) and the tank is near death and can't flee, or is being lagged by bashes/trips/bodyslams and so on.
The problem isn't with rescue, it's with people not using it properly, such as rescuing someone who is fighting a group and lagging them, making them die. Maybe an option to toggle if you can be rescued or not in combat (autorescue or something) that can be turned off if you want, so you can't be rescued - maybe not ideal, as there are times you will want to be rescued in combat and times when you don't.
The idea of not having any lag for the person being rescued if rescue fails is already there. The person being rescued/person rescuing are lagged if the rescue works. If rescue fails, only the person rescuing is lagged.
Dey