Early 80s British mountain bikes used BMX bars and stems, along with long headtubes in order to get a high posture similar to that of motorcycle trials riders. The idea was that with the rider standing on the pedals, the rider can quickly shift their body weight and manhandle their way through nasty bits of technical singletrack.

http://www.flickr.com/photos/38236150@N06/3516370418/

The other effect of the high posture was that it simultaneously unweighed the front wheel, making it easier to hop it when encountering rocks or mud, and weighing the rear wheel, which has the effect of improving traction in the mud.
http://www.flickr.com/photos/38236150@N06/3515560693/

This setup can be changed by rotating the bars forward, which has the effect of turning the handlebars into pseudo-Bullmoose handlebars.
http://www.flickr.com/photos/38236150@N06/3516371042