Kufu - 5:58 pm on Aug 23, 2008 (gmt 0)

It's a question of stability.

If you've been operating with a title for a long time and your rankings have been built on that particular combination then changing the structure radically would result in such a change.

As has been said before, if the site is an authority or otherwise very 'mature' then this is less likely to be a problem.