Its all a matter of preference. Some people like the look of Mac's and dont like OS X but are used to Linux. Honestly I think OS X is just fine and in many ways a lot better than Linux when it comes to what OS for a Mac. Granted ive not used Yellow Dog, but I know people who do and mostly they have it to play with and learn Linux, not to replace OS X. You gotta keep in mind that OS X is built off BSD so its still BSD under the hood. You can still do 99% of things you could with BSD in OS X via the "Terminal". Hell, for the server, a majority of configuration is command line only, not a whole lot through the GUI. At the seminars on hacking ive spoken at, a couple of times there were some OS X machines and guys running exploits off them. Hell at DC last year we had a girl running a Mac doing a good amount of Offensive work on the team I was on.