You know, the last whisper items are kind of garbage.
Its not that they aren't good vs armor stackers-- and I understand the whole point of them being a niche purchase that is only good vs armor stackers-- its that they aren't significantly better than something like bloodthirster.
You could buy a last whisper item and have significantly more damage vs the enemy tank...
or you could just buy bloodthirster and have moderately more damage vs everyone, including the tank, on top of some crazy lifesteal.
Last whisper is kind of like thornmail-- it is basically only effective against one person on the enemy team on average, so it doesn't get bought often. Infrequently you'll fight a team that has 2 or 3 squishy auto attackers and buying a thornmail makes sense.
The thing is, even if they had 2 or 3 tanks, I still wouldn't feel good about last whisper. At that point I'd rather have steraks gauge or another lifesteal item.
If its just going to kick one person's ass, it really needs to actually kick their ass. But I don't like the idea of making armor effectively worthless for tanks, and I especially don't like the idea of giving assassins the ability to kill tanks just by buying a single item. It needs to have something on it that is better for auto attackers than assassins, and it needs to have some downtime so it can have a brief "oomph" of power.
If the last whisper items had an active that could be used to do some minor true damage on hit for 6 seconds (like between 20 and 40) i could justify taking it over other items against tanks.
are unstoppable if you dont though. (and a huge pain in the ass regardless XD not that i'm complaining since that's the point of them)