In counter-insurgency, you are just trying to take out the bad guys - although taking them out usually helps. Counter-insurgency is also about winning the hearts and minds of the populace. While 500-pound bombs with precision guidance can kill bad guys - and have been used by the United States and Israel in Iraq and the Gaza Strip - they also cause a lot of collateral damage. That tends to cause
PR problems.
This is where the lighter, slower, COIN aircraft come in handy. The lighter weapons they use can often be aimed more precisely, usually due to the slower speed of the aircraft. These weapons are much less likely to cause collateral damage. A standard Mark 82 500-pound bomb usually carries about 200 pounds of high explosives. That is a pretty big bang - and in addition to taking out the room a sniper or machine-gunner is in, it tends to destroy the rest of the building and damage neighboring buildings. A 2.75-inch Hydra rocket usually only has a 15-pound warhead. That is usually enough to do the job, and it won't bother the neighbours that much.
One other benefit that new counter-insurgency planes would bring to the table is permitting the use of precision-guided missiles like the Hellfire. That missile has a range of about 8km, can be laser-guided or use millimeter-wave radar. The Hellfire has a 20-pound warhead. Currently, the Hellfire is primarily carried by attack helicopters, which have been vulnerable to RPGs and surface-to-air missiles. COIN aircraft are faster than helicopters, and harder to get a bead on. COIN aircraft can also get to a given place faster than helicopters, which can only go about 300 km/hr at most.