The point of writing software in assembly is exactly the same as writing an OS (or even a kernel): fun and self-education. I mean, they probably don't mean to take over the current mainstream x86 operating system with MenuetOS, I believe it's more about fun and learning. The same is true for writing code in assembly; it is a lot of fun (at least for me) and you learn a lot about how the underlying architecture of your chosen platform really works.