I really think you're underestimating the importance of weapons. They've put a pretty heavy emphasis on them in all the interviews. I really don't think they'll be limited to a cosmetic purpose. Is there any info you've seen that would indicate that?
I don't have a clue what the developers will
do, but I do know exactly what they could
do, and they could design the game in such a way that your concern does not exist. In addition, I think it is fairly likely that any lack of creativity will not be the result of the job system.
Item stats and slots are entirely arbitrary. Developers can create as many slots as they wish. Any slot can hold an item with any stat attached to it. We have rings that give +50 mp and breastplate that give +50 defense, but we could just as easily have rings that give +50 defense and breastplates that give +50 mp. From this it's easy to say that, if desired, developers could use an item (weapons as in FFXIV) as the method of selecting a job and then create a new slot--say a knee slot for hilarity--and give knee pads that exactly duplicated the stats of what would normally be weapons.
As for the reason I believe it is likely we will not see the issues in your concern, it is specifically because weapons have been said to be so important. They're so important that I believe they won't be like other items. I don't think we'll see axes with +str or +acc like we'd see on a belt, ring, or glove. If anything I think you'd see job specific bonuses. For example if there were cesti (making you a monk), then you wouldn't see stats like +str or +acc on the cesti, rather you see stats like +kick damage or +counter chance. Since no other class has kicks or counter, then there is no reason for them to want this item even if they could wield it.