Because there is system Ruby on OS X and otherwise there is no choice unless you use something else, so at that point you just leave it blank... but for Debian/Ubuntu and Fedora/Redhat you might probably either supply your own or use system Ruby provided by apt/yum on your server unless it's a multi-tenant situation where rvm (or others) would be better suited (and even that is subjective since nothing is stopping you from pulling in 1.8, 1.9 and 2.0 on Debian.).

It doesn't harm to leave it, but it does harm not to mention that there are at least better alternatives for dev if you need to think about multiple Rubies, I vote leave it, but do as @parkr implied (or ...)