Is it situational at all? I've seen people say they built it when they saw some doomed carries stick around waiting for Doom to end so they could jump back into the teamfight. I don't see it as core, but it seems it might useful in some cases.
And are you being sarcastic about lvl? Death? I'd say 20% of max hp is pretty strong and you can always check the scoreboard for hero levels. I've also read that some Doom players keep some spare skill points so they can quickly level it up to the appropriate level to do the 20% dmg. I really don't think it is a joke, especially after I saw a bristleback that was destroying my team get his hp melted when he was 6-slotted.
I agree that it costs alot of mana early, unless you buy some mana items, you won't be able to cast all 3 of your spells in a gank, so that could be problematic. Would that make agh's somewhat better on Doom? It gives some decent mana and I don't recall building many int items apart of shiva's.