6000 fuel!? Does that thing ever need to land?
Depends on the flight profile but for instance a Darkstar based on Europe can stay airborne for about 72 hours over South America. Now it is was possible to assign it to automatically patrol an area then it would be perfect to watch over a region.
EDIT: I've actually even increased the fuel to 15000 units since the craft's speed has been reduced to 750 and the time it takes to reach its patrol area actually can cut a bit of the time patrolling.
It is an interesting craft to use. Besides patrolling regions for days you can also send it after a UFO that just flew out of ground radar coverage and it can redetect it quicker than an interceptor. The drawbacks are the max speed which doesn't allow it to keep up with fast UFOs and refueling takes a while.