This is also known as self-archiving or Green Open Access and it can serve two purposes:
providing access to researchers who otherwise would not have access either because they don't belong to an institution with access or can't acquire a subscription to the journal
allows Author's to keep track of and access to their works
Short answer: The goal of a digital repository it to house content and not just serve as a list of citations.
Long answer: Statistics. Downloads are easy to track and can be seen on your Author Dashboard. Once you upload a pre-print or post-print you will be able to get usage statistics based on downloads. Currently, there is no easy way to track usage when a user clicks on "Link to Full Text" and even the hard way of tracking doesn't guarantee that they looked at your article - only that they looked at the record page and saw the citation information & abstract.
Other considerations: In the open-access world of the Digital Commons Network, users have the expectation of finding Full-Text. Users may abandon their search if they can't easily retrieve the article/work that they want.
