The applications that use a search-field-based approach, with or
without a sidebar, are applications that are for
reading/browsing purposes mainly, and not for writing:
Preview, Safari, iTunes, Mail’s main Viewer window, etc. So why
did Microsoft choose to copy that UI instead of sticking with the
dialog-box-based approach that appears to be the preferable option
for writing tools?
The generous answer would be to say that Microsoft is trying to
innovate and bring to writing tools the simplicity/intuitiveness
of the search UI used for reading/browsing tools.
The less generous answer would be to say that Microsoft simply
didn’t really think the whole thing through.