The original reason for the question was I have a fairly good thing going with my current domain but a keyword specific domain became available. So I really was wondering if it made sense to build that out (which I decided to do).
Imagine you have Miriamstuff.com and it's doing well raking 15th for "Blue Whirly Widgets" However you can't seem to break to the top. A few of your pages do well "Blue Whirly Widgets with handles" etc. Then one day you have an opportunity to buy BlueWhirlyWidgets.com... do you take it?
In my example it's real estate specific. So the domain was a larger city option outside our metro area.