I don't think anyone is suggesting one build Linux from scratch and then use it as their primary OS.

The value of LFS is not in having the system you build, it's in understanding it. After you've read and worked through the book, you've managed to produce a functioning GNU/Linux OS, and presumably you know what all the parts are.

From there, understanding any published distribution is a matter of understanding what makes it unique, maybe a different package manager or init system, or different userland packages. Regardless, the fundamentals still stand, and your ownership of the system is improved by having worked through the book.