If you mean pre Ni ninjutsu and you have not seen a NIN tank ever, then you really have no right to say that they cant tank with no Ni spells. Ichi only NINs can still tank, and pretty reliably, usually you invite another WAR or MNK so they can help tag team tank the mob so you give the NIN an extra 6 secs or so to cast it. It goes faster than most other tanks as well especially with double NINs Utsuing, and even WAR/NIN with a NIN. Their shadows absorb alot of attacks therefore rendering the NIN invulnerable. With Slow Paralyze and Blind, there is more room for the NIN to manuver about(and with Haste/Elegy later, that is even more so).

In case you didnt know, NINs use Utsusemi to tank, it gives them Copy Images which absorb one attack. Some moves go through Utsu like a bomb toss but leaves shadows untouched, but some moves like Aerial Wheel removes all shadows yet leave the NIN untouched. The NIN uses Provoke, Elemental/Enfeeble Ninjutsu in conjunction with Utsusemi to tank. Should the party be flawless in what its doing the NIN will lose 0 hp from any attack except those AoE exceptions. If they do foul up they have to take hits like a man until their shadows are back up. There is a Utsu spell at lvl 12 which has 3 shadows and long cast/recast, and one spell at 36ish i believe that has 1 sec cast time and a moderate recast and 4 shadows, with the right enfeebles and evasion equipment for a NIN the shadows can last a while. In case one of em dont work you have to juggle the spells, and remember that Ni(2) > Ichi(1), in case they cannot overwrite their other shadow should Ichi need to be casted.