But what do casters cost?

Would you rather have Sizemore or two warlords equivalent to Vinnie? Would you rather have Maggie or 50 Goblin Knights? Would you rather have one lookamancer or 700 bats?

Sure Sizemore is powerful in his element, but that's a huge risk to put him out in the field. He makes the GK tunnels nearly impassable, but most people wouldn't risk taking him out of the city, and if Ansom has just stormed the walls like the coalition wanted, Sizemore would have been pretty much worthless. He was useful in one single niche, so Parson played his enemy to ensure that he ran to where Sizemore's strengths could actually help.