The policy runs counter to the growing calls for more open access which often comes from researchers and academics but not publishers. It would seem that rather than think of a potential solution that looks forward taking full advantage of current media culture and technology the AHA has chosen the historical approach. In the process, they have attempted to, at a very late date, turn back the clock on scholarly publishing.
That would then lead me to conclude that any young researcher and/or PhD student would view the association as not one which they would want to actively participate in. Ever. And that's history for you.
In their announcement they cut straight to the chase:
The American Historical Association strongly encourages graduate programs and university libraries to adopt a policy that allows the embargoing of completed history PhD dissertations in digital form for as many as six years. Because many universities no longer keep hard copies of dissertations deposited in their libraries, more and more institutions are requiring that all successfully defended dissertations be posted online, so that they are free and accessible to anyone who wants to read them. At the same time, however, an increasing number of university presses are reluctant to offer a publishing contract to newly minted PhDs whose dissertations have been freely available via online sources. Presumably, online readers will become familiar with an author’s particular argument, methodology, and archival sources, and will feel no need to buy the book once it is available. As a result, students who must post their dissertations online immediately after they receive their degree can find themselves at a serious disadvantage in their effort to get their first book published; it is not unusual for an early-career historian to spend five or six years revising a dissertation and preparing the manuscript for submission to a press for consideration. During that period, the scholar typically builds on the raw material presented in the dissertation, refines the argument, and improves the presentation itself. Thus, although there is so close a relationship between the dissertation and the book that presses often consider them competitors, the book is the measure of scholarly competence used by tenure committees.