Late to the thread. I've heard of Nashville Numbers, but never learned the system or used them. I just went and browsed through the opening pages of the Hal Leonard book on Amazon that's been linked. And it looks to me like it is more than just writing 2-5-1 instead of ii-V-i. It has ways to indicate song form, chord inversions, and "odd bars" (varying time signatures from the main one for the song) in a pretty concise fashion, something you can get on a single page rather than needing to spread it over three pages of sheet music in standard notation. I can see it having practical value in certain settings.