Is there any truth to this claim: "Keep space on your hard drive"

Is there any truth to this claim: "*Keep space on your hard drive.* Your
computer’s primary startup drive (typically the C: drive on a PC or the
Macintosh HD on a Mac) should be no more than 60% full at any time. Filling
that drive up until it’s almost full brings on a world on instability in all
programs, and Lightroom is no exception." (
http://thelightroomlab.com/2009/07/optimizing-lightroom-for-best-performance-top-ten-lightroom-speed-tips/
)

I see it made frequently in the photographic world - yet it seems to be
complete BS to me. I can understand a warning that you should keep 10-20 GB
but - 40%? Why.

Cheers
Mark