The current weapon categories (as of the 2011/07/05 blog post) have some that will inevitably be densely populated (blunt weapons or swords) while others are separated for no obvious reasons. I see Thomas was quick to react to the blog comments, so here's some more, hopefully useful. :)

Flails are a very specific weapon that shouldn't have a whole category named after them. They are essentially a shaft with a chain and a heavy ball or cylinder hanging on it. What makes them different from typical blunt weapons? The chain, which is something that increases their range and impact force at the cost of a decreased degree of control. So I'm thinking that they should rather be organized as chained/segmented weapons, which all share the same basic trait, and more such weapons should be created. It shouldn't be hard to come up with a decent number of them aside from flails: nunchuks, meteor hammers, three section staves (these things: http://j.mp/ifIAOx), and so on.

Slings should probably be combined with whips (as "Slings & whips") due to three major reasons: 1) the possible amount of different whips/slings seems incomparable to most other categories; 2) nobody is going to invest a lot in either of them due to their extremely niche use; 3) their methods of operation are somewhat similar, with the main difference that a sling uses an external projectile, while a whip's body is continuous. Not sure how it's planned in JADE, but in real life conditions, both are about equally useless in close combat.

Unless you're planning to add lots of different types of bows and crossbows or otherwise differentiate their usage, they need to be combined in the same category. If you, however, plan to add other mechanical/automatic weapons, they should belong to the same category crossbows are in.

Should one-handed polearms really be a separate category? To be honest I'm having troubles imagining anything larger than a short hunting spear or a javelin used together with a shield, let alone a different weapon.