Really? Does a Mumak need to fly? The new scenarios mean that most of the time a big monster can start in the middle of the table. Half the scenarios require models to be conveniently in close proximity and stay there in order to capture an objective or the high ground, and getting the most out of a banner requires the same thing...let's all bunch up around the dude with the flag. It's harder to get away from a non-flying monster now. If your opponent has a dragon with fire are you going to want to build a shield wall with spear support and a banner all clumped together or do a concentrated cavalry charge within 15" of something that can kill all models (including multiwound ones) within 2" of it's target on a 3+. It's evil and doesn't care about shooting into a fight. You know that the retribution is going to hurt if you form a solid battle line, so you'll either do everything you can to take the Dragon down or you'll dance around attacking piecemeal and less effectively. Either way it'll make you change your battle plan without flying or even breathing fire. Go on, charge your Dwarf Shield wall with WoMT spears into my goblin line, I'll peel back my line of spears and breath fire into the combat. It'll hurt you more than me. Spread your line out to avoid the fire and I'm getting more 2 vs 1s than you, and usually more dice to roll = more win..... then I think I'll save the Dragon's will.
If it's not flying then it may be less tempted to do a heroic move, so there's more might you can use for the courage tests.I do think fire would be much better if it counted as a thrown weapon. A dragon breathing fire on some enemies, then charging into combat with others would be so cinemaic. A mighty hero is standing in the shield wall with his warriors, facing his enemies. The Dragon swoops down breathing fire, the hero's Fate saves his life and he stands defiant as all around him are consumed in flames. The Dragon charges into the hero and they fight in single combat. Epic.