necro pet tank is more of an offtank, helping out player maintanks in those "oh shiet" situations ... I wouldn´t rely on pet maintanking even if I had a whole group of necros his aggro is worse and not that predictible and can die very quickly by some unlucky big hits, too
also Frenzy CD is 60 seconds, but CD of the General of the Dead (who basically keeps the pet alive) is longer, so you can´t cover more than one minute (20+20+20s) with 3 necros