There is a place for this. Check out the binoscopes in these threads:
http://www.iceinspace.com.au/forum/s...ad.php?t=24892 (great construction notes for a 12" bino)
http://www.iceinspace.com.au/forum/s...ad.php?t=25742 (includes pictures of the 10" at the Snake Valley star party).
These scopes use diagonals as tertiary mirrors to fold the light path, so that two dobsonian optical trains can be used in effect as a set of binoculars. However this is a niche application, & you wouldn't use a diagonal on a conventional monocular dobsonian for the reasons stated earlier in this thread.
Phil