In the case of server-side state saving, implementation of ResponseStateManager
saves a new state for an every request,even for a same viewId as previous was,
and generates a new value for "javax.faces.ViewState" hidden field. That feature
allows JSF to restore proper view state then user returns to previous page by
browser "back" button and submit some form from that page again.
But that behaviour is incorrect for AJAX requests because in this case browser
does not save page in history ( Even if "back" button support for AJAX will be
implemented, it could be done by special 'iframe' object only, and does not save
page in history anyway ), and old values is only going to garbage. As a result,
small view states cache ( which default size is 16 ) could be refilled by values
from AJAX requests soon, and browser "back" button support will be broken.