Bindside, I guess that it's because it isn't really legal to carry real weapons?

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Well, today my instructor said something about this subject... He thinks that you should only learn weapons when you master the unarmed part of the art because then you'll be able to understand it and you'll be able to actually do something useful with it... Also, a weapon is an extension of your limbs and if you can't even properly use your limbs, it will be quite difficult to master a weapon.
But I guess this only applies to Pencak Silat because our jurus (kihon/kata) can be used both armed and unarmed.
So, if you first learned all 36 jurus, you could fight quite effectively without a weapon - and then you add a weapon, like the cabang (sai), teach the few techniques you need to properly use it (turning it around etc.) and sudddenly you can do 36 jurus with a cabang without too much difficulty.
If you were to put a cabang in someone's hand and teach him the jurus that way, you won't have the benefit of understanding both armed an unarmed...
Also, the jurus represent a load of principles - it's not like you learn one jurus and then you suddenly master it - it's all about being able to use all the principles and techniques of one jurus in all sorts of combinations and situations. I think you can't possibly teach that with a weapon, you'll really need to master the unarmed part first if you really want to understand your weapon and use it in your style.
I mean, I think it's fairly easy to pick up a sai and start twirling it around. But if you're a karateka and want to learn how to use it according to the principles in your art, it will be quite hard to learn that principles by using the weapon immediately - you should rather learn how to do karate first...
My teacher, however, doesn't think that you need to master all jurus before even touching a weapon... He's taught us a few basic techniques of the stick and the sai because then we'll be able to practise them and eventually, when we really start on the weapons work, we'll no be hindered by not knowing how it works at all... Still, it's not like we've got a fixed curriculum or so and there's so much to learn about the unarmed part alone, that weapons have always had a more inferior place in our training - there's just not enough time to cover it all.
So, when should one start with weaponstraining? As soon as you understand the unarmed part of the art... I'm not saying that it has to be perfect, but I think that it's quite useful when you understand how to move, block, evade etc.
I also practise Iaido and I think it's really hard to train a weapons art without a weapon

. So, in that case, weaponstraining should begin on the first day - of course.