Erebus can down out grunts pretty easily with bat swarm, it is effectively an AE
I occasionally use bats as an AoE, but only in emergancies, or there's no forseeable danger for at least another 15 seconds or more, factoring in mana usage. I (and smart Erebus players) would rather try to keep it for situations where it's use is integral.
What I think really hurts Erebus most is that, especially at early levels, Swarm already takes up a lot of mana. This big mana increase might just kill the combo. Which is a shame, because, quite frankly, what does Erebus do after teleporting?
Also, escaping with swarm will be harder too, as, once again, you'll be drained in mana. Personally, if you never get low on mana, you're not extending yourself far enough(likewise, if you end up running out, you end up overextending). I think this will drastically change Erebus' safe energy threshold, and in a bad way.