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][1]

 - 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.

  [1]: https://releasenotes.docs.salesforce.com/en-us/winter17/release-notes/rn_lightning_slds.htm