I agree with everybody - I wouldn't have realized that my game was glitched out on Mac if my brother, who has a Mac, tried it out.
Of course, you can install a virtual machine to test your game out on other platforms, but if you have a Mac like my brother, you would have to pay the hundred dollars or so to buy a copy of Windows.
Another problem I have with building to multiple platforms is that, at least for Unity, building to multiple platforms is so painful, having to switch the platform, wait 3-5 minutes, build, wait 3-5 minutes, and then repeat three or more times.
Fortunately, I made a tool that automatically builds for me, with minimal technical settings like SuperUnityBuild. You can download it here. :)