I could see a 200 rank cap if there were only ever going to be the current Weapon Classes. Once you add in that they are going to "Eventually add all the weapons from XI" as they said in beta, that starts adding up a lot. Then you have the Job System which i'm hoping is just a series of jobs with 10 job levels each with each level granting a individual bonus. Because having to have a job and weapon class up to 200 is extreme.

To be honest though if you look at FFXI all the weapons had 300+ Ranks. (FFXIV the Weapons ranks are the same thing basically.)