It takes way too much CPU resources to use mainstream, especially as people have gone to 4MP and 8MP cameras.
NVRs have done it that way for years. BI was behind the times.
If you run just continuous then it will only use the mainstream for everything. Unless you have a screaming machine and few cameras, watch your computer choke LOL.
But if substreams missed anything, none of us would use it. Our priority is good video of a perp.
But with the AI in the cameras so good, unless you have a bunch of cheap cameras or older ones without AI, there really isn't a need to use BI motion anymore except for certain situations, like tracking animals or for LPR.