The biggest reason I can see so far for not updating to Yosemite on Macs is Boot Camp compatibility. There's something in the Yosemite installation process that affects partition mapping ("Core Storage") and can cause issues.
In my case even a test Yosemite install to an external drive affected my volumes to the point where I cannot currently install Windows 8.1 via Boot Camp without first doing a complete wipe and re-partition on my MacBook Pro.
On the other hand I've read posts in other forums where people had no problems at all with their Boot Camp installs. Anyone have suggestions or experiences with this?