I thought the whole reason behind using ball screws was to eliminate all backlash and the reason they are chosen for CNC machines?
Applying drag to the table via the gibs seems counter-productive to me. Much like driving a car with one foot pressing on the accelerator and the other pressing the brake at the same time. 
Willie,
you
may want/need to apply some drag - not if you're fully CNC, but if you're trying to use a ball-screw equipped machine without the CNC kit to drive
and hold the axis, as summarised by others above.
On my manual (still ACME) mill, unless I'm taking really big cuts or really need to hold tight tolerances, I can get away with just winding the handles to the correct location and leave it there, without locking X and/or Y - courtesy of the significant force needed to back-drive an ACME/trapezoidal screw.
On a ball screw machine, cutting forces - or even just vibration - can cause a ballscrew axis to walk - effectively the screw is back-driven just like an Archimedes Drill. A CNC machine will either have this steadied by the holding current in a stepper or actively kept in place by the encoder closed loop on a servo.
Concerning the situation where you are using ballscrews on a manual machine:
1/ Applying a little drag to a free-running manual ballscrew axis might be useful to get a more controlled cut (I sometimes use a second hand to apply a little drag to the cross or compound slide on the lathe), esecially to stop it being pulled away if you're climb cutting.
2/ If you didn't have your hands on an axis, I think you'd need more drag or to lock it, as any significant machining would be likely to cause the untended axis to move - at least it did for me!. I only have 3 full axes on my ballscrew-equipped CNC mill but at one time had a jury-rigged 4th. 'A' axis which had its own motor but used the 'Y' axis control, so the program moved into XY position, then stopped and I threw a switch/relay which changed the Y axis drive from the Y motor to the A motor. On one occaision, I forgot to lock the Y axis before press cycle start again, and the cutting forces back-drove the Y axis ballscrew/motor....
Dave
p.s. and regarding driving with one foot on the accelerator and one foot on the brake, that's how
John Haugland taught me to use left foot braking in a rally car!