Will manned aircraft have to defend ground targets against unmanned aircraft in the future?
Some advantages of UCAVs over manned aircraft in the strike role:


The enemy doesn't gain hostages when they get shot down over enemy territory
The numbers that can be deployed are limited by the rate at which they can be turned out on automated production lines, not by the time it takes to train a combat fast jet pilot to a high standard (about 5 years minimum)
The UCAV can potentially be much smaller than a manned aircraft, if a reduction in range is acceptable, reducing the radar signature and vulnerability to air defence systems
As a result of 3, the UCAV can potentially be based nearer the area of operations and launched by catapult or using a rocket booster, leading to a shorter reaction time 


In a scenerio where manned aircraft are intercepting UCAVs it is probably reasonable to assume that the UCAVs will have an advantage in numbers. UCAVs engaged in tactical missions will probably operate at low level and rely on superior manouverability and short range missiles to defend against attacks by manned fighters.