UAVs such as Reaper are potentially very useful in a CAS role. They have persistance and a good ISTAR capability which may preclude positive ID/RoE issues associated with handing off a target between assets.
On the flip side, they are generally more susceptible to weather factors, are bandwidth hubgry and ironically are fairly manpower intensive to operate right now. Their biggest weakness is that they are relatively slow; you cannot retask a Reaper from Northern Afghanistan to a troops in contact call in Helmand in the timescale required. You also cannot employ one for a show of force!!
In short, armed UCAV such as the MQ-9 compliment but as yet do not replace manned CAS and other fires.
Regards,
MM