Yes why ? If you look at the latest Steam Survey, Mac OS X only 3.23% of the users base on Steam so you would think that a Mac version is just a waste of time.
Except if you live in California. Pretty much every friend I have has a MacBook. And let's be honest : pretty much everyone who has a decent computer in California has a MacBook. California is Apple's birth place and playground and they own the market. And it's not just for laptops : it's the same for smartphones and tablets.
There's also another group of people who loves Apple products : college students and journalists meaning the people who are going to play your game and the people who are going to talk about it.
So swallow your long time PC gamer pride, find a MacBook and make a Mac build. There's a Mac version of the Unreal Engine 4 Editor and you can export a Mac build as easy as a Windows build.
And Linux ? There's even less people using Linux. It's only 1% of the users on Steam and it's fragmented between the different distribution and even between the different versions of the same distribution. But there's two factors to consider :
-with the release of the Steam Machines this year (they come with SteamOS), Linux might finally get some attention
-Linux fans are pretty devoted and welcome any new game on their plateform
Same as Mac : there's a Linux versuin of the Unreal Engine 4 Editor and you can easily make a build. If you have a PC with Windows or a Mac, you can setup a dual boot to install SteamOS or Ubuntu 14.