Necro and Manna Robe

[Vallanor]

Can anyone explain to me how a necro and a Manna Robe work in tandem? I always assumed the hp to mana ratio of their best life tap dot was better than the hp to mana ratio of the robe, but now I'm not sure that's the case. According to the wiki, the best necro lifetap dot has a 2.24 hp to mana ratio (1110 hp for 495 mana) vs the 3 hp to mana ratio on the robe. It seems like casting Vexing Mordinia then using the Manna Robe until it wears off would actually result in less mana over time than just sitting in lich form.

I know a few necros who have one and love it, so I'm probably just missing something. If it's used in "oh crap" situations where you need to regen mana fast or get killed and other situations where the long-term efficiency doesn't matter, I can see it being pretty awesome. Is there another use?