3 ravagers shots clicked in a triangle pattern will still kill things pretty well, or significantly eat into Protoss shields/health to help in subsequent engagements. It will do this for an effect that is more pronounced and less expensive (mana and units lost for Zerg in process) than the old chain fungal ever did against anything that wasn't a sentry. With smart casting on Ravager we're not talking about 1 fungal 1 Ravager shot, you can spam that ability. It's not like it just procs an AOE DOT that doesn't stack (like Fungal). Every hit will do full DMG; 60 damage is still 60 damage, and combining the ability with fungal (or even without) will give Zerg more Zone control over an engagement than they ever had before.
Fungal also does 30 damage over 4 seconds, so unit hit by both will end up taking 90 damage over a short period of time, and that is a hypothetical 1 fungal, 1 ravager shot. Let's say a second shot this the same place 1 second after the first (or both hit at the same time), that's 150 damage from 1 infestor and two ravagers. That's half the damage of a Ghost Nuke (300 damage to units), and the ravager shots re-gens almost instantly.
I think we'll see Ravagers getting a lot of use. If not in ZvT, ZvP and ZvZ.
EDIT: Random thought: Immortal is now going to have an ability that can help protect it from emp and widow mines since it gains extra shields from the new active.