By Carbuncle.Shokox 2011-03-18 08:09:41
I believe at the time (before the Merit Point system) they simply didn't want NIN to be the best enfeebling person at the time, so they purposefully left out Jubaku: Ni, and even the San line of Ninjutsu (until they added them via merits in a shitty fashion). The devs of the past left plenty of magic out of the game across all jobs in retrospect for preserving "game balance".
New spells must follow a Ichi -> Ni -> San path, that's something they won't change for the sake of Ninja at all. It's based on casting time anyways.
Also comments like saying "SE lacks common sense" or "it blows how they design this ***" is so sorely out of date-- you can't compare the old fashioned hard-headed devs of yesteryear (2002~2009) who set the rules and had a very hard time communicating with the player base with the ones that run the game currently (2009~2011+) and are extremely open with the player base. If it matters, I remember reading back in the day that those same devs mentioned NIN would be "overpowered" if they had A+ skills (keep in mind the permanent 75 cap at the time, due to dev stubbornness). All you have to do now is request that they fix up this stuff via Twitter or the new FFXI forums, and you'll have the greatest chance of seeing a change.
All that said, it would be cool if they added Jubaku: Ni and even San enfeebling spells.