What is this option? Why it isn’t checked by default?
Default is checked because of a lack of tests on it?
It has received quite a bit of tests (It was introduced about a year ago), but it will take some time before it becomes the default. I personally switched to V3 months ago and I’m happy with it!
I’d very much like to make it the default at this point, but AnkiDroid doesn’t support it yet, and that could cause confusion for new users.