No, Nin is not the only sub. But in most cases, its the best.
/Nin ---> Its really the default. You're going to want this because it adds a great deal to your tanking ablilities in the form of a little extra evasion, shadows, limited enfeebs, more reliable snk/invis. Keep in mind that shadows make it so you dont have to walzt as often, which means you can spend that TP on other things.
/Sam ---> For those times when you already have a tank or the mob you are tanking has fail Acc. This is more for pure DMG and support healing. Sekka, meditate and conserve TP are the real gems here. Although, well timed macros and gear swaps make Seigan/3rd eye become like a defensive TP move if you do it with the right staff, but its very situational. When using /Sam, I keep a terra staff, twilight torque and have fan dance ready to proxy tank in case the mob we are thumping gets a little unruly.
/Thf ---> Spike DMG, hate control and drop control. Self explanitory.
/Mnk ---> I dont have any experience with this sub, but counterstance and dodge would seem to be good tools for tanking, but your defense would be a strong as a level 10 whm's.
/War ---> Pure DD. You will just plain hit harder and double attack. Not really much info needed here.
/Drk ---> DD support, no tanking. Attack bonus, soul eater, last resort, and many of the absorb spells are very useful. Although you wont be dishing out as much DMG as WAR with this sub, with a little thought you can do some cool stuff. Note that you get absorb tp at 45, absorb agility adds a little to your subtle blow and you have access to the stun spell too. This setup, like Drk as a main, is very timing/setup intensive. I only use this when fighting mobs with MP, for aspir samba and already have an established tank. But I would keep this as more for fun time, because your drk magic skills will only be at level 45. One of the reasons I didnt mention drain and aspir as those are very skill based spells.
/Mage jobs ---> pretty much for soloing or teleing. lol