Because there are less components installed on a Windows Server core OS, it needs less patches than the full OS. I read in several places that therefor it needs less reboots after patching.
I'm running Server 2012 core in production since September 2012 now and I don't remember a single patch-Tuesday when I did not have to reboot the server after installing Windows updates.
Are there any hard numbers out there that compare the required reboots for core vs. Full OS?
Less reboots may be the main reason why people choose to go with Server core. If it actually requires just as many reboots as the full OS install, they may think again the next time they set up a server.
It is rare - but it happens. I remember patch days without reboot on server core but with reboot on a regular server.