Its not pointless at all, really, and actually it fits perfectly into what a tank is designed to do.
In general (and there are a lot of exceptions and nuance) the rule of thumb is that a Tank has high base stats, lower ratios, and their job on the team is to 1) start fights, 2) soak damage and 3) protect their allies, usually by locking down or pushing out enemy threats (whereas Supports more commonly protect allies by directly buffing the ally themselves, such as with heals, shields, or speed boosts)
The other key part of a tank is that they typically deal lower-than-average burst damage, but can output steady, consistent damage that, over the course of a fight, can become a threat if left unchecked. A tank that can't output any form of damage gives the enemies no reason to focus them, and a tanks entire job is to wave their arms shouting "HIT ME! HIT ME!"
Nunu, for example, gets attention with his Ult. Its a huge area, it deals massive damage, but it has a very long channel time. This isn't meant to weaken Nunu just for the sake of power checking him. Its meant to give Nunu a tool to say "HEY EVERYONE LOOK AT ME". Any CC the enemy uses on a tank, is CC they aren't using on a Carry.
Alright, that's all pretty much LoL 101 and you already know all that. I just like talking a lot and setting the stage. The whole reason that Zac's passive works so well (in addition to its thematic cohesion), is that it says "HEY GUYS ATTACK ME, NOT THEM!". When a Zac splits into bloblets, the enemy team naturally wants to kill them precisely because he's an easy kill at that point. If he dies in the middle of a team fight, it forces the enemy between two options: focus the rest of the team (which lets Zac revive, potentially with 1/2 of his probably-quite-large health pool), or to stop and focus Zac's blobs (which, being 4 targets, may require either using more abilities or more people attacking them), and if they do then he is still doing his job of distracting the enemies from his allies. So overall, its a perfect passive for a tank. It punishes the enemy, whether they don't focus him, so he can deal more damage/knockups, or if they do, which draws heat off the carries.
So what about Aatrox? Someone else mentioned him. He has a similar passive, but if you notice, it does completely different things. Aatrox's health is not determined by whether or not the team focuses him during his "undeath" state. Instead, the health he gets back (in order words, how useful his passive is likely to be) is determined by how much damage Aatrox himself was able to do in the fight that lead up to him dying. As a Fighter, this fits perfectly with his ideal play pattern. He's supposed to be a threat, and becomes more deadly as he gets more and more blood in his "Bloodwell", ramping up his damage even as he nears death. Plus, Aatrox is invincible during his revive, which is key. Zac's passive focuses around the time when he's reviving, whereas Aatrox is about what he does once he's revived.