I have not personally tried the Resqme. I have practiced with the Gerber Hinderer, a spring loaded center punch, flashlight, tire iron and a couple of the hammer style window punches. The spring loaded metalworkers center punch works great. The Hinderer too. The tire iron and light bounced off several times before we broke a window. We broke one of the cheap hammer type tools that you see "As seen on TV". The other worked ok after a few tries. A pick head axe works great

I wouldn't hesitate to pop a window with the hilt of the Hinderer though.
Trying to break a window with a Maglight takes lots of effort. Most times more than is really practical. The hard sharp point is what you need.
The easiest way to release someone from a seatbelt, is the latch. Law enforcement prefers it too as they like to inspect belts and know if they were being used in the accident. Lots of questions are asked if you cut the belts.
As for the windows. Try to roll them down first. The glass mess hinders operations.
I have cut belts from patients a couple times. I have punched a few rear windows when opening a top for extrication. Most often the cutter pops the window when you cut the "C" post. A pick head axe does the job sometimes too, since we need to punch a hole to cut the windshield anyway.
Having a tool is better than not. You will have the keys so you will have that tool. I don't see it as a bad idea. Better than trying to kick out a window for sure!
I could see it working in a rescue situation. How about this scenario: Car in a canal or lake. Victim unconcious (diabetic or cardiac). You dive in to rescue. Doors are held by water pressure or sides of canal. You need to pop the rear or side window. The rear window is preferred as it will not shower glass on the victim. Jumping out of you vehicle you take your key ring tool.
I could see it working.
That scenario happens more often that you might think. I can recall a couple in the last few years around here. I don't know what they used to break the window. It probably wasn't that tool or we would have heard it mentioned.