I really don't want to get into an argument that relies on fallacy, but...

If the concern about ninja sharing accessories with other strength-based classes is perceived to somehow unbalance the classes, it fortunately does not. Already strength based classes compete with each other accessories, so the fact that another class comes in sharing the same accessories does not reduce the chances of said accessories from dropping. Those same accessories still have a similar chance of dropping just as before and the chance of obtaining said accessories still remains based on entirely on group composition (at least 20%). The eight-player full party could have four monks, or four dragoons, or four ninjas and in the end, it wouldn't matter how many strength based classes there are.There could be two, there could be one, there could be three. Unless the players themselves form an alliance for themselves consisting of 6 melee dps, the typical, balanced group (through duty finder or otherwise) generally won't see a change in overall drop rates, or chances to obtain said items. Players already compete with each other for chances to enter a group, through duty finder or elsewhere, this doesn't really change just because a new class enters the mix. Secondly, high-level accessories are already extremely rare drops. Most accessories are acquired through currencies, while most weapons are acquired through drops from boss monsters. This is unlikely to change in the short-run.

What will change, is the drop rate on weapons and armor. Like all other physical DPS classes, it is likely the Ninja will get its own set of gear at the level 50 range, in addition to seeing its own weapon drops in the level 15-50 range. In this particular realm, do the chances of players picking up new equipment decrease. But it would decrease regardless of the class entering the game. And in the long run, as more and more classes get introduced, more and more weapons and equipment needs to be introduced that can cater to them, especially when it comes to weapons.

As an aside, I'm currently going over the new video footage of the ninja from the anniversary video, lot's of great new information to discuss on here. Will post later.