Is it necessary to export features with a version number?
It's metadata for keeping track of a Feature release. In D7, the use of Features has two cross purposes:


Bundling Drupal configuration up as a part of a released feature that can be deployed to any site (e.g. a bunch of re-usable content types). 
Snapshoting config as code which is useful for capturing config that D7 didn't normally have (and has been supplanted by D8's config management system).


In the first case, the use of version tagging is more useful when you're a vendor of some project leveraging Features. A tag gives you an identifiable way to know what will be rolled back if you force a Feature back to its original state.

If you're using Features solely as config as code, versioning is not really important. Especially, if it's part of a large project that is already leveraging some release tagging.