If only one in ten provinces turns out to have a fire site, which gives only +2 gems, than for an investment of 20 gems you get +2 fire, just as good as unnerfed dom2 clams
A bit more seriously, its just tedious to shuffel mages around all the time, and you can't have your pretender marching with your army all the time (because that will be too slow for early expansion, and too dangerous for later battles)