I think you either misinterpreted Itachi, or didn't see what he was really trying to say, but than again Kishi could have written it wrong, or translation could be off to. Kishi recently has always made some sort of mistake in explaining things.
Itachi said in the same page, that Izanami is considered Kinjutsu. Kinjutsu = forbidden techniques which shouldn't be taught or used. As of what we know there can be two ways you can look at it:
1. A jutsu that does harm to the user.
2. A jutsu that breaks the law.
Multiple shadow clone is consider a kinjutsu (harm to oneself), simply due to the fact that it can bring any user to the brink of death becasue of the amount of chakra it uses (other than Naruto). This tech. shouldn't be taught or used, but yet we see Naruto using it.
A jutsu that breaks the law would be Tobirama IWR, as it requires a human sacrifice to resurrect the dead. We still see this kinjutsu being used by Oro and Kabuto.
Death Seal Reaper could go in either one, as it not only harms the user (killing him), but the person its used on, but we still seen this used by Minato and third.
Izanagi/Izanami effect the users, reason why it is a kinjutsu, but shouldn't be used in fight, but we seen Danzo/Tobi use Izanagi and Itachi now use Izanami in battles.
I believe this is what Itachi spoke of, other than Naruto if anyone else used Multiple shadow clone, they will be really low on chakra, and on verge of death pretty much. Izanagi/Izanami are the same, the users one eye sight is gone and if countered well than the user is pretty much screwed.
If Izanami is used to counter Izanagi, and Tobi/Danzo used Izanagi in battle, but Izanami can't be used in battle? How would that make any sense? Or rather how would that be a counter if not able to use in battle VS Izanagi user?
This is why I don't think Izanami is useless in battle, but rather better to be used in a battle VS someone with Izanagi to counter it and/or punish them as stated.