I don't understand tagada's point, but i can see myself using mercies if they are like this.
Gunships are slow and vulnerable to AA, while fighter bomber tend to be able to snipe stufff without taking too much of AA damage. There's also the element of surprise. Random grey triangle isn't a scout, it's a mercy sniping your t2 mex, while gunships are more easily spotted, alone or in group.
On a big 20x20 maps, i can definitly see those mercy replace gunships when the ennemy has 2-3 AA turrets or few MMA to protect.