I've done some research on this, but honestly really can't find an absolutely hands down compelling reason for locating the system databases off the c: drive where the OS resides.
In my environment
- Space on c: drive is not an issue.
- I don't find navigating through the file system to system databases ( and log files) a big deal or cumbersome.
- As I understand it, these system databases ( with perhaps the exception being temp) don't really get much use, are not "write and or read" intensive, say as compared to "User"
So moving them to another physical disk with better performance characteristics doesn't seem to make much sense to me, and or worth the trouble to be perfectly honest.