Zerg's Problems:
1) Dealing with Siege Units.
A) Extra Broodlings take tank fire, as opposed to actual units in ZvT. Vikings aren't in range when you are massing broodlings. Marines can't engage near broods as fast because of meatshield. Overall, extremely useful.
2) PvZ: Blink Stalkers have a hard time blinking under broodlords when there already is broodlings. Void rays can't kill broodlords while they're massing up. Roach Broodlord Infestor becomes even more ridiculously viable. Overall, Extremely useful.
IMHO: The Broodlord is just way better then the ultralisk and with these micro tactics it is going to become a neccesity of the zerg army, like the infestor.