Hasn't this been done before in the
"US to deploy JDAM-equipped Super Tucanos" thread?
There's a presentation which would imply that the Tucanos are being bought to make (a massively expanded) FAC training and retraining programme somewhat cheaper. Just think how many flying hours are required to give every MFC and FOO in a single Battlegroup a go of "working with CAS" during pre-operational training, let alone the amount for a deploying Brigade...
USN Irregular Warfare presentation (pdf) - look for the "Imminent Fury" explanation on slide 21.