GP age - They're pretty hit or miss for me. I probably bring them with more often then not though.
If there's multiple sections of walls I need to get through I usually will.
If there's an anti tank gun near the edge but a mortar is covering that part of the wall.
If I'm expecting my archers will need to go through an exterior wall I'll save one so they don't get stuck. - This one is pretty rough because you have to time it right, if there's nothing left nearby inside the wall he'll just attack buildings while my archers spend the rest of the time shooting down 3 sections of wall.