/NIN used to be the go-to subjob, offering offense (Dual Wield) and Defence (Shadows/Suble Blow).
Now, with the addition of Dual Wield III to THF, /NIN only offers Shadows, Subtle Blow and Sneak/Invisible.
The only two spells that are "must haves" for /NIN to function are Utsusemi:Ichi and Utsusemi:Ni. You will also want to pick up Tonko: Ichi/Ni and Monomi: Ichi as well for the convenience of invisible and sneak.
Some of the other elemental Ni spells may be rarely situationally useful for weakness triggers, but for the most part, all other spells that /nin offers have no use as a sub job.
If you are looking for a place to start, you need: Shadows (Utsusemi Ichi and Ni), Invisible(Tonko) and Sneak(Monomi).
At some point you will also want to level /WAR (best DD sub) and /DNC (Cheapest/most effective fodder farming solo sub)
Other situationally fun subs: /SAM, /MNK, /DRK
In regards to your other question, dedicated tanking is somewhat of a lost art in FFXI. The enmity system is broken and has been for a long time. The developers seem to recognise that this needs addressing so maybe we'll see a fix in our lifetime. Then again, maybe not.
Having said that, THF was not really designed with tanking in mind, even though it can to some effect in various ways.
What strangers and shout groups really want from you: Treasure Hunter.
If DD is all that is required (TH not effecting the event), most groups will not seek out a THF to fill that role. THF is not specifically a BAD DD, but jobs like WAR, MNK, SAM, DRK etc all have higher DD potential without all the positional restrictions and job ability timer bottle necks that THF has to deal with (not to mention attack and accuracy deficiencies).
That is not to say that you can't murder faces on THF. It's just harder and takes more time, not only to learn how, but also to put together gear sets to make it work. Their are thousands of "TH only" mule characters and bad THVs out there. Which is probably why most of the player base thinks the job sucks.....most of the Thieves they see all suck.
It is possible to be a great DD, Low man tank and valuable contribution to any event on THF, but you will constantly face playerbase perception issues when dealing with pick up groups.