The slide/page pane in impress and draw are important there as it provides
navigation between pages/slides and the primary means of inserting pages/slide.
Writer isnt in the same boat as navigating to another page is as simple as
scrolling down and inserting new pages is as simple as pressing ctrl+enter or
pressing enter a few times.
The page pane is common in pdf viewers which have no editing capabilities, so
the content isnt constantly flowing from page to page and then the thumbnails
would have to be constantly refreshed. I've also seen the page pane in WPS and
iWork, though with the ability to zoom out and see 10+ page thumbnails at a
time, would it be that useful?
Agree with stuart that it is a useful UI tool for those who would be interested
in using it and that it shouldnt be active by default, just like it isnt in any
other word processor that has this feature.