Looks like createLink() is limited to the scope of controller/GSP/taglib (anything with a request). But it is a useful general-purpose tool, and there is really nothing specific to a request object required.
e.g. in a MailService, you may want to embed links in the email, and createLink() /createLinkTo() is the obvious choice to do that.
In short, createLink/createLinkTo() should be available in global scope.