In Exam Ref 70-331: Core Solutions of Microsoft SharePoint Server 2013 one can read the following quote:
“When you are adding new keywords, you might see this message: The Site Does Not Contain A Default Keywords Term Store. This occurs when you have not yet selected the default storage location for keywords within your SharePoint environment.”
If default keywords term store isn't configured, where are enterprise keywords stored (if they are stored at all)?
If there is no default storage of keywords, than none can be saved. You'll always end up with an error which guides you to check the checkbox of default keyword term store in one of your Managed Metadata Services.
The first one (where you check to see if Default Keyword Term Store is null or not) ties back to a Managed Metadata Service (MMS) Application configuration option. There’s a checkbox on the service application’s property page (the property page for the indented one) that controls whether or not this is the default keyword term store. If that’s not checked and this is the only MMS application available to your farm, site.DefaultKeywordsTermStore will always be null.
Reference: Correctly Provisioning Managed Metadata (Taxonomy) Fields
