Overall it seems to be a nice new feature, but as you say, not very user friendly to activate fast. Maybe there a workaround yet to be discovered, or a firmware update will resolve that.
Another issue is you have to use Canon software at this time to extract the Roll to get to the CR3 files you want, hopefully, eventually you will be able to use other software editors to do that. Once again more steps to get to your CR3 file.
On a scale of 1-10 a 5 for user friendly. The best thing I could see it for is, when your waiting for a bird to fly from a perch, you will get that series of shots, so long as you keep that shutter 1/2 down prior to releasing shutter .
I agree with your assessment. Perched birds taking off is a great use case. Or perhaps a heron about to lunge on a pray, etc. Anything where you are waiting for action to take place, but you don't know when it will happen exactly and you can't just start a burst early because you might fill up the buffer before the action takes place. Another example could be a baseball batter where you can have focus on the batter and wait for the swing to press the button.
A couple of notes on usability.
1. In DPP I didn't find an easy way to extract all the frames you are interested in from a sequence. You can set a begin frame and end frame and then trim off the rest to generate a reduced sequence so you don't have to store the irrelevant frames before and after the action you are interested in. However, there was no way to automatically extract all the images in the sequence into individual .CR3 files. You have to manually do it one by one. A bit painful. I also hope once other software packages start supporting this it will be more user friendly.
2. The camera seems to be constantly writing to the card while you half-press the shutter button. This can potentially chew up your SD card's endurance and/or put its indirection system into a poor state for the rest of the shoot. I haven't heard anything about this anywhere, but something I worry about.



