A better alternative for this is to use extends="force:slds" in our lightning applications. Firstly it will include the latest version of library and it would take away the hassles of developers updating the versions now and then.
Reference document : Extends force:slds
- Communities templates & lightning experience have inbuild slds support.
And if we incorporate extends force:slds in our lightning applications we can keep up to date and be consistent with Lightning Design System enhancements.
Update :
Though these versions number will be of use when we use lightning design system in VF pages without lightning components. Hope Salesforce comes up with some tag similar to <apex:includeLightning /> to ease this use of being consistent with the lightning design library static resources in VF pages.