Like Vawn says, /nin and definately nin is not broken. Also it's relatively well known by the better players that a full party of /nins is less efficient than dd's subbing dd subjobs.
Yes it is broken. A defensive mechanic whereby players regularly mitigate all incoming damage is stupidly broken. Most MMORPGs, including FFXI, are not built to handle such a mechanic. It becomes impossible to scale encounters You need to scale mob damage to make encounters increasingly difficult for traditional tanks, but that has almost no effect on blink tanks. If you attempt to scale attack speed for blink tanks then you aren't increasing the difficulty so much as the risk.
Broken doesn't necessarily mean overpowered. It means something doesn't mesh well with the game mechanics.
I hear what your saying, but /nin doesn't help as much as you think thar.
Shadows are wiped with any AoE magic, and the recast on Utsusemi is enough to balance the mechanic. It's also pretty widely known that ninja tanks can't hold hate for jack. If all is what you say it is in FFXI, EVERYONE would sub ninja regardless of job and paladin tanks would be obsolete.
The ONLY situation with many /nins is a merit party, and even then its HIGHLY preferred to merit with a bunch of 2 handed weapon uses subbing /sam.
You guys are probably remembering FFXI from when dual wielders /nin were king, but after a large 2-hander buff a while ago that made 1.5 dex = 1 Acc and 1.5 Str = 1 Att, Sams Drgs and G.axe Wars are the merit party champs and only gain 3 shadows from /nin, but from /sam gain Hasso (Increased acc + 10% haste on 2 handed weapons), Seigan/Third Eye (1-5 Shadows) which unlike Utsusemi will block all hits of a multi-hit attack like Pecking Flurry from Colibri, and Meditate (when subbed instant 60 TP).
So really, You are thinking of FFXI in a very old mentality and while you'll still see people using /nin because they're lazy, in anything that matters you'll most likely see real sub jobs.
Also, even for soloing /nin is a dinosaur. Dancer is the new solo subjob. Sam/Dnc, Pld/Dnc, Nin/Dnc, etc use tp for curing themselves.
Sorry for the long as **** post, but if /nin was broken you wouldn't be allowed to go to events without it, and I never attend an event subbing ninja except on Ranger.