Then there's also the situation where Mavericks was unusable on your machine because of graphics card driver kernel panics, and Apple won't do anything about it because your machine is out of warranty and they insist that is must be diagnosed as a hardware issue.

And so you look at the Yosemite beta and you're like “it's late in its dev cycle. It takes 1-2 years to put these OS updates together; we're at the 90-95% mark right now.  It's basically done.”  And you notice the trend of odd releases (10.5 Leopard, 10.7 Lion, 10.9 Mavericks) being more experimental in architecture and more unstable, and the even release (10.6 Snow Leopard, 10.8 Mountain Lion, and 10.10 Yosemite) being architectural refinements of the previous release.  And then there's that fancy-shmancy new software that only runs on 10.9+ because we live in the kind of world now.

And so you say “yeah, okay, I'll try Yosemite on a second partition.”  And everything works smoothly, much more smoothly than that upgrade to Mavericks you had to painfully back down to Mountain Lion out of.  And you throw caution into the wind — caution you've seen no tangible evidence of, just murmurs of the word “beta” — and upgrade before the final release.  The end.